# Effect of environmental conditions on the ageing behaviour of 3Y-TZP grinding media

A Pacquelet, Laurent Gremillard, D Bouttes, N Benameur, J Chevalier, V

Garnier, H Reveron

► To cite this version:

A Pacquelet, Laurent Gremillard, D Bouttes, N Benameur, J Chevalier, et al.. Effect of environmental conditions on the ageing behaviour of 3Y-TZP grinding media. Journal of the European Ceramic Society, 2025, 45, pp.117677. 10.1016/j.jeurceramsoc.2025.117677. hal-05231764

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# Effect of environmental conditions on the ageing behaviour of 3Y-TZP grinding media

A. Pacquelet<sup>a,b</sup>, L. Gremillard<sup>a</sup>, D. Bouttes<sup>b</sup>, N. Benameur<sup>b</sup>, J. Chevalier<sup>a</sup>, V. Garnier<sup>a</sup>, H. Reveron<sup>a,*</sup>

## ARTICLE INFO

**Keywords:**

- Low temperature degradation
- Y-TZP
- Mehl-Avrami-Johnson
- Residual stresses
- Ageing kinetic

## ABSTRACT

This study explores the ageing behaviour of 3 mol.% yttria-stabilised tetragonal zirconia (3Y-TZP) grinding media under different environmental conditions, focusing on temperature and pH effects. Accelerated ageing tests in an autoclave, monitored by XRD analysis, were modelled using a double Mehl-Avrami-Johnson (MAJ) law, revealing two transformation stages. Raman microscopy identified residual compressive surface stresses in 3Y-TZP beads, explaining the two-stage ageing and their positive ageing-retarding effect. Ageing immersed in liquid water or water vapour produced similar results. The study also examined the impact of different media and pH levels on ageing kinetics using digestion vessels. Acidic environments slightly slowed the ageing process, while basic environments, especially sodium hydroxide at pH=11, accelerated it. Ammonia did not alter the ageing behaviour compared to water. The dissolution of alumina in basic solutions significantly influenced the ageing behaviour, discussed in terms of OH⁻ activity, oxygen vacancies, and alumina doping.

## 1. Introduction

Yttria-stabilized zirconia, especially 3Y-TZP (i.e. ZrO₂ + 3 mol.% of Y₂O₃), is widely used for its excellent mechanical and physico-chemical properties [1,2]. Indeed, a phase transformation-toughening phenomenon can occur under stress, when metastable tetragonal (t) zirconia transforms to monoclinic (m) zirconia at the crack tip. This phase transformation goes along with 3–5% increase in the lattice volume, leading to superimposition of a compressive stress field around the crack tip, that can slow down or even prevent crack propagation, thereby increasing the toughness of the ceramic. Therefore, 3Y-TZP shows outstanding mechanical properties [2,3], including a fracture strength that may be higher than 1000 MPa and a toughness of 4–6 MPa·√m [4]. It also presents excellent corrosion and wear resistance and a relatively high density of 6 g/cm³. All these properties make 3Y-TZP ceramics widely used for wet-grinding media.

It is well known that 3Y-TZP is sensitive to ageing when exposed to a moist environment within a temperature range from room temperature to 400 °C [5,6]. This phenomenon known as Low Temperature Degradation (LTD), or ageing, is characterized by a spontaneous *t-to-m* phase transformation at the ceramic surface [5,7,8] that may result in

micro-cracks, grain-pull-out and deterioration of the mechanical properties [9–11]. Although several mechanisms were proposed in literature, the diffusion of water species (i.e. OH⁻ ions) through oxygen vacancies, proposed by Yoshimura et al. [12], is the most widely accepted as responsible for LTD in 3Y-TZP. The decrease in the number of oxygen vacancies (filled by OH⁻ ions) and the accumulation of internal stresses in the lattice explain the spontaneous destabilization of the tetragonal phase. Most of the research work on LTD of Y-TZP for biomedical applications was performed in autoclaves. ISO 13356 Standard based on ageing test at 134°C - 2 bars with water vapor, allows an increase of the monoclinic fraction of less than 25% after 5 h of treatment (corresponding to ~20 years *in vivo* [13]).

On a microstructural point of view, the LTD phenomenon in Y-TZP ceramics can be described as a thermally activated nucleation-growth of monoclinic zones in the tetragonal matrix. The evolution of the monoclinic phase content could then be described as following a Mehl-Avrami-Johnson (MAJ) law which reflects the nucleation-growth (N-G) mechanism, and the parameter b (see Eq. 2) is thermally activated according to an Arrhenius law [8]. Typically, for 3Y-TZP ceramics activation energies around 80–120 kJ/mol have been reported [4,8].

<footnote><sup>a</sup> INSA Lyon, Universite Claude Bernard Lyon 1, CNRS, MATEIS, UMR5510, Villeurbanne 69621, France</footnote>

<footnote><sup>b</sup> Saint-Gobain Research Provence, Cavaillon 550 Rue Alphonse Jauffret, Cavaillon 84300, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: helen.reveron@insa-lyon.fr (H. Reveron).</footnote>
$$ \alpha = 1 - \exp(-(b \times t)^n) \quad (1) $$

$$ b = b_0 \exp\left(\frac{-Q}{RT}\right) \quad (2) $$

where $\alpha$ is the degree of transformation, $n$ is an exponent related to the nucleation / growth ratio, $b$ the inverse of the time $\tau$ necessary for $\alpha$ to reach $1 \cdot e^{-1} \approx 0.63$. $Q$ is the activation energy, $R$ the gas constant and $T$ the absolute temperature.

The MAJ law is derived from a nucleation-growth law (see Eq. 1) of a new phase (monoclinic) inside a stabilized structure (tetragonal + cubic). Additionally, studies can be found in the literature where a two-step phase transformation can be observed in some crystallization experiments where a transient metastable phase could exist [14] or two different phases could coexist and crystallize successively [15]. In these studies, a double MAJ law was used corresponding to two MAJ equations summed together, each of them corresponding to a distinct N-G process. This double MAJ law is given in the Eq. 3.

$$ \alpha = \alpha_{M1} (1 - e^{-(b_1 t)^{n_1}}) + \alpha_{M2} (1 - e^{-(b_2 t)^{n_2}}) \quad (3) $$

Where $n_1$, $b_1$ and $n_2$, $b_2$ are constants respectively associated with the first step and the second step in the phase transformation associated with the nucleation mode ($n_1$ and $n_2$) and the nucleation and growth rates ($b_1$ and $b_2$) [8,16]. These constants are system specific. Similarly, $\alpha_1$ and $\alpha_2$ are two constants representing the saturation volume content in each part of the double N-G law, with $\alpha_1 + \alpha_2 = 1$.

In the field of wet-milling process, the ageing of Y-TZP zirconia beads has not been extensively studied, even though it is known that these grinding medias can be used for long periods of time (i.e., several hundred or even thousands of hours) in contact with aqueous suspensions at relatively high temperatures (typically 70–80 °C). In these conditions, the ageing phenomenon may lead to accelerated wear [17]. Moreover, relatively few articles have considered the effect of the pH on LTD [18] even though the suspensions to be milled are rarely neutral, but rather acidic or basic. Sato et al. [18] did not observe any significant effect of the pH in aqueous solutions on the ageing behaviour of 3Y-TZP. In their study, Sato et al. used sealed stainless-steel tubes in which they placed Y-TZP pellets and solvents. However, unlike autoclave ageing tests, this method does not allow monitoring of ageing kinetic of Y-TZP over time, resulting in a single ageing value per condition. Since they only did one single ageing test of 120 h for each solution, it leads us to question again the effect of the medium on the ageing kinetics of 3Y-TZP.

Thus, this study aims to evaluate the stability of 3Y-TZP grinding media under various temperatures and pH levels, thus simulating wet-milling conditions. LTD kinetics will be analysed based on MAJ laws to determine activation energies and gain insights into the main transformation mechanisms. The impact of pH (from 2 to 12) and of the nature of the pH-modifying agents on LTD will be examined. Additionally, the effect of the release of residual stresses on ageing will be discussed.

## 2. Materials and methods

The wet-milling ceramic beads investigated in this study were processed from a ~3 mol.% $Y_2O_3$-stabilized zirconia (Zirmil® Y, Saint-Gobain/Zirpro, France) with a narrow bead diameter distribution centered at 0.8 mm. Table 1 summarizes the beads main characteristics.

During production, the beads undergo a polishing step which can induce surface stresses due to plastic deformations localized at the bead extreme surfaces. In order to study the effect of surface residual stresses on LTD, some Zirmil® Y beads were annealed at 1200 °C for 12 min (here after referred as “stress-annealed” beads) according to [19,20].

<table><caption>**Table 1**

Composition and properties of Zirmil® Y grinding medias.</caption><thead><tr><th rowspan="2"></th><th colspan="3">Zirmil® Y</th></tr><tr><th>ZrO<sub>2</sub></th><th>Y<sub>2</sub>O<sub>3</sub></th><th>Al<sub>2</sub>O<sub>3</sub> &amp; Others</th></tr></thead><tbody><tr><td>Chemical composition (wt%)</td><td>93</td><td>5.5*</td><td>1.5</td></tr><tr><td>Density (g/cm<sup>3</sup>)</td><td>6.0</td><td></td><td></td></tr><tr><td>Vickers Hardness (HV<sub>1</sub>)</td><td>1250</td><td></td><td></td></tr><tr><td>Grain size (nm)</td><td>330 ± 51</td><td></td><td></td></tr><tr><td>Crystal structure</td><td colspan="3">≈ 99 %vol stabilized phases**</td></tr><tr><td>Shape - Size (mm)</td><td colspan="3">Spherical - 0.8</td></tr><tr><td>Beads size distribution (D<sub>90</sub>-D<sub>10</sub>)/D<sub>50</sub></td><td colspan="3">0.058</td></tr></tbody></table>

*3.1 mol.% $Y_2O_3$. **stabilized phases includes both tetragonal and cubic phases.

### 2.1. Ageing procedures

Two devices were used to study the accelerated ageing of the beads, i.e., a classical autoclave and a digestion vessel. A summary of the different ageing tests is given in Table 2, describing the beads used and the conditions of ageing for each parameter studied.

#### 2.1.1. Autoclave

The ageing kinetics were evaluated in water steam at three different temperatures (120, 134 and 145 °C) using an autoclave (LA-MCS203, SANOclav, Bad Überkingen, Germany) under ~1, 2 and 3 bars for different durations up to 420, 110 and 51 h respectively, depending on the temperature. For each temperature, 40 g of Zirmil-Y beads (≈ 30 000 beads) were aged. At several ageing times, the autoclave was cooled, and around 1 g of beads (≈ 700 beads) were collected every 5 h and dried for X-ray Diffraction (XRD) characterization. The remaining beads were returned to the autoclave for further ageing. To study the effect of stress annealing, 40 g of stress-annealed Zirmil-Y beads were also aged in autoclave at 134 °C following the same procedure.

In addition, a supplementary test was carried out in autoclave at 134 °C for 100 h, with 40 g of beads immersed in liquid water (in a glass beaker full of water) and 40 g of beads in water vapor (in a beaker at the top of the autoclave, over a grid).

#### 2.1.2. Digestion vessel

Digestion vessels (Anton Paar, Graz, Austria) with 45 ml PTFE cups were used to investigate the effect of the chemical nature and pH of the ageing medium. On the acidic side, two solutions prepared with nitric acid (J.T.Baker, Nitric acid 69.0–70.0 %) at pH= 2 and 3 were used. On the basic side, three solutions were tested: one prepared with ammonia (EMSURE, ammonia solution 25 %) at pH= 11 and two others with sodium hydroxide (Fischer scientific, Sodium hydroxide pellets, 97 +%) at pH= 11 and 12. Deionized water was used as neutral pH medium. The concentration and pH values of the solutions and water are also reported in the Table 2.

The ageing behaviour at 134 °C of Zirmil-Y beads (15 g) immersed in 15 ml of different solutions was evaluated in the digestion vessels. For ageing, once filled and closed, four different digestions vessels (i.e at pH= 3, water, and two pHs= 11) were placed in an oven at 134.0 ± 0.3 °C for treatment sequences of 5 h each. After each thermal treatment step, the digestion vessels were cooled down in cold water and zirconia beads were carefully rinsed with deionized water, a few were removed for XRD characterization while the remaining beads were returned to the vessel for another ageing sequence with a renewed solution. In a second experiment, ageing in more acid (pH= 2) or basic (pH= 12) solutions was evaluated by placing simultaneously the two digestion vessels in the oven at 134 °C. The pH of the solutions was measured before and after each ageing treatment sequence (pH-meter pH150, Thermo Fischer Eutech™, Waltham, USA). The pH evolution (ΔpH = pH<sub>final</sub> – pH<sub>initial</sub>) of acid and basic solutions was estimated and the average of the pH evolution over the complete ageing cycles reported (ΔpH<sub>average</sub>). The pH of the deionized water was not monitored during
<table><caption>**Table 2**
Ageing tests conditions, initial state of the beads, and characterization conducted.</caption><thead><tr><th>Parameter</th><th>Device</th><th>Zirmil Y Beads</th><th>T (°C)</th><th>Medium pH (mol/L)</th><th>Ageing steps (h) x number of steps</th><th>Single step ageing test (h)</th><th>Analysis</th></tr></thead><tbody><tr><td rowspan="3">Temperature Ageing kinetic (Q, n)</td><td rowspan="3">Autoclave</td><td rowspan="3">New</td><td>120</td><td>in H₂O steam</td><td>10 × 42</td><td>-</td><td rowspan="3">XRD</td></tr><tr><td>134</td><td>5 × 55</td><td>-</td></tr><tr><td>145</td><td>1 × 51</td><td>-</td></tr><tr><td>Surface residual stresses</td><td>Stress-annealed</td><td>134</td><td>in H₂O steam</td><td>5 × 15</td><td>-</td><td>XRD</td></tr><tr><td>Positioning of beads</td><td>New</td><td>134</td><td>in H₂O steam</td><td>-</td><td>100</td><td>XRD</td></tr><tr><td rowspan="10">Medium pH</td><td rowspan="10">Digestion vessel</td><td rowspan="10">New</td><td rowspan="10">134</td><td>immersed in H₂O</td><td>5 × 31</td><td>40°; 75°</td><td>XRD</td></tr><tr><td>HNO₃</td><td>5 × 34</td><td>75°</td><td>ICP°</td></tr><tr><td>pH= 2 (1.10<sup>-2</sup>)</td><td></td><td></td><td></td></tr><tr><td>pH= 3 (1.10<sup>-3</sup>)</td><td></td><td></td><td></td></tr><tr><td>immersed in H₂O</td><td>5 × 34</td><td>40°; 75°</td><td></td></tr><tr><td>pH= 6-7</td><td></td><td></td><td></td></tr><tr><td>immersed in NH₄</td><td>5 × 34</td><td>40°; 75°</td><td></td></tr><tr><td>pH= 11 (2.3.10<sup>-1</sup>)</td><td></td><td></td><td></td></tr><tr><td>immersed in NaOH</td><td>5 × 34</td><td>40°; 75°</td><td></td></tr><tr><td>pH= 11 (1.10<sup>-3</sup>)</td><td>5 × 31</td><td>75°</td><td></td></tr><tr><td>pH= 12 (1.10<sup>-2</sup>)</td><td></td><td></td><td></td></tr><tr><td rowspan="2">Reproducibility of the method</td><td rowspan="2">5 identical digestion vessels</td><td rowspan="2">New</td><td rowspan="2">134</td><td>immersed in NaOH</td><td>-</td><td>75</td><td>XRD</td></tr><tr><td>pH= 11 (1.10<sup>-3</sup>)</td><td></td><td></td><td></td></tr></tbody></table>

these tests because of the difficulty for stabilising and measuring it.

As a reference for pH variation during a sequence of 5 h, 15 ml of NaOH solution at pH= 11 were placed in two different vessels without adding beads. One of the vessels was placed in the oven at 134 °C. After 5 h, the pH was then measured in each of the vessels to assess its evolution without beads at 134 °C and room temperature.

Four identical vessels were used simultaneously in this study for ageing tests. To ensure that each digestion vessel behaved in the same way and to study the reproducibility of the ageing tests, 15 g of Zirmil-Y beads were placed in each of these four vessels, immersed in 15 ml of NaOH solution at pH= 11 and heat-treated together in the oven at 134 °C for 75 h. The volume fraction of the monoclinic phase (XRD) and its standard deviation was estimated.

### 2.1.2.1. Assessment of Leaching during ageing.
Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES, Agilent 5800, Agilent, Santa-Clara, USA) analyses were performed on selected solutions at pH= 2 and 3 and pH= 11 and 12, before and after ageing of the zirconia beads, to follow a potential ionic leaching from the beads. 20 ml of solutions were placed in each vessel with 30 g of Zirmil-Y beads. The vessels were then put in the oven at 134 °C for 40 h or 75 h (see Table 2). The different solutions were collected after the test and analysed by ICP-OES to evaluate their Zr⁴⁺, Y³⁺ and Al³⁺ ions content. Five ICP measures were performed for each solution (40 h and 75 h ageing) the average contents of Al, Y and Zr were reported (Table 6). In all cases the standard deviation was less than 1 %. Compared to the tests performed to study ageing kinetics, larger volumes of solutions were used to obtain enough liquid to perform the ICP analyses. Similarly, more beads were added to the vessel to increase the amount of potential leaching that might occur.

## 2.2. Characterization methods

The surface of the beads was observed using Scanning Electron Microscopy (SEM, SUPRA, Zeiss Ltd., Oberkochen, Germany) performed on new (as received) and on aged beads. In some selected samples, the evolution of the microstructure below the surface was followed on polished sections. Ionic polishing was used for cross-section preparation (ionic polisher, ILION II+, Gatan, Pleasanton, USA) by means of a double ion gas gun at an angle of incidence 90°. The energy of the gun

was set at 6 keV for 4 h. Moreover, zirconia grain size was evaluated following the ASTM E112 standard through the linear intercept method and applying a corrective factor of 1.56 to report the real mean grain size.

The fraction of the monoclinic phase on the surface of the beads was analysed by X-ray diffraction (XRD, AXS D8 Advance, Bruker, Billerica, USA) using Cu-Kα radiation, a scan range of 2θ 26 – 33°, with a scan speed of 0.6°/min and a step size of 0.04°. The zirconia beads were bonded to a carbon film and rotated during measurements at 30 rpm, so that an average intensity value was obtained, as illustrated in Fig. 1.

The X-ray penetration depth in zirconia for Cu-Kα radiation is around 5 µm [21,22]. Due to the particular shape of the samples (i.e. bed of spherical beads of 0.8 mm diameter), the analysed surface was not perfectly flat. Consequently, Rietveld refinement analysis was not applied to precisely estimate phase composition and/or lattice parameters. The monoclinic zirconia content was calculated according to Garvie et al. [23] and Toraya et al. [24] equations:

$$X_m = \frac{I_m^{(111)} + I_m^{(111)}}{I_m^{(111)} + I_m^{(111)} + I_t^{(101)}} \quad (4)$$

$$V_m = \frac{1.311 \cdot X_m}{1 + 0.311 X_m} \quad (5)$$

where $X_m$ is the integrated monoclinic fraction, $I_m^{(111)}$, $I_m^{(111)}$ and $I_t^{(101)}$ the area under the (hkl) peaks of monoclinic (m) and tetragonal (t) zirconia

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Scheme of the sample preparation for XRD analyses.</figcaption></figure>

<footnote>* For chemical elemental analyses by ICP-OES</footnote>
phases and $V_m$ the calculated monoclinic volume fraction (average over the penetration depth of the X-rays).

In order to estimate the stresses at the surface of new and annealed beads, quantitative mapping was performed by Raman spectroscopy (Raman microscope, LabRam Soleil from HORIBA, Kyoto, Japan). All the spectra were recorded at room temperature. The laser used has a wavelength of 532 nm, a power of 32 mW. The pinhole confocal aperture was set at 200 µm. A surface of 30 µm in diameter located at the top of the beads was analysed, using a 1 µm probe size to collect spectra with a 0.5 µm spacing. The exact position of the tetragonal zirconia peak (wavenumber around 146 cm⁻¹) was then measured to evaluate the residual stress [25] inside the tetragonal crystals using the following linear relationship [25,26]:

$$ \Delta\nu = 3\Pi_{\nu}\sigma_{p} \qquad (6) $$

where $\Delta\nu$ is the shift of the tetragonal peak position with respect to 146 cm⁻¹ (reference position for the “stress-free” zirconia tetragonal peak), $\sigma_p$ the average normal stress in the tetragonal phase and $\Pi_{\nu}$ the uniaxial piezo-spectroscopic coefficient which is −0.6 cm⁻¹/GPa for the 146 cm⁻¹ tetragonal band in Y-TZP [25]. A peak shift towards lower wavenumbers indicates the presence of tensile (positive) stresses, whereas a peak shift towards higher wavenumbers implies the presence of compressive (negative) stresses.

## 3. Results

### 3.1. Effect of temperature on ageing

The evolution of the amount of monoclinic phase transformed during ageing tests at different temperatures is shown in Fig. 2.

Fig. 2 shows that $V_m$, initially close to 0 %, increases with ageing time but does not reach 100 %, which is consistent with the fact that the material exhibits a certain percentage of non-transformable cubic phase (7,8,13). The maximum value of $V_m$ reaches a saturation at $V_M= 71$ vol % regardless of temperature (meaning that the initial cubic content was in the order of 29 vol%). Therefore, the degree of transformation $\alpha$ given in Eqs. (1) or (2) can be expressed directly as:

$$ \alpha = \frac{V_m}{V_M} \qquad (7) $$

<figure><img src="image_4.png" /><figcaption>Fig. 2. Monoclinic content $V_m$ as a function of time at 120 °C, 134 °C and 145 °C. The blue dotted lines correspond to a single MAJ law and the red solid lines represent the fitting law obtained with a double MAJ law.</figcaption></figure>

Interestingly, the curves cannot be fitted well using the sigmoidal single MAJ function (dotted blue line). Indeed, the increase occurs quite clearly in two stages. Initially, the slope of the curve is relatively low, until it reaches a slope inflection at around 15 vol% of the monoclinic phase. Then, $V_m$ increases rapidly before reaching the saturation at 71 vol%. Increasing the temperature of the ageing test accelerates zirconia ageing without significantly affecting the two-stages shape of the curves.

The single MAJ fitting with linear regressions of $ln(ln(1/\alpha))$ versus $ln(t)$ using the whole experimental data set show poor correlation coefficients of ~ 0.93 for an average slope of n ~ 1.5 at each temperature, see the dotted blue lines in Fig. 3 (left).

However, with the poor linear regression of the single MAJ, an activation energy Q was obtained by plotting $ln(b)$ versus 1/T. The value of Q is estimated to be 129 kJ.mol⁻¹.

The double MAJ law given in Eq. 3 better fits with the experimental data, as it is shown in Fig. 2 and Fig. 3 (red continuous lines). The mean quadratic error between the values obtained with the double MAJ and the experimental measurements is estimated at 3.3 %, considering the three tested temperatures. In contrast, using a single MAJ equation in a mean standard deviation of 6.6 %, i.e. two times higher. The numerical fitting values of the parameters in this equation are given in Table 3, and are deduced from the plots of $ln(b1)$ and $ln(b2)$ versus 1/T with good correlation coefficients.

SEM observations of cross-sections near the surface of the beads aged at 134 °C for 60 h show a clear boundary between non-transformed and transformed zirconia, as illustrated in Fig. 4. This contrast difference between the monoclinic and non-transformed grains is attributed to the twinning effect that occurs within the monoclinic grains during the t-m transformation. The average zirconia grain size was estimated at 330 ± 51 nm from SEM images.

In the transformed areas, intergranular microcracks appear at some grain boundaries and are most often parallel to the surface of the beads.

### 3.2. Effect of annealing

It should first be noted that the annealing treatment results in a similar phase composition (i.e. the monoclinic, tetragonal and cubic phases) to that of the new beads. Ageing tests in autoclave at 134 °C on annealed beads are shown in Fig. 5. Fig. 5a reveals that after annealing, the ageing kinetics does not follow the same trend as those of the as-received beads since the previously slow t-m transformation observed up to 15 vol% of monoclinic was suppressed by this thermal treatment.

As shown Fig. 5b, the kinetics can be well fitted by a single MAJ law, where the evolution of $ln(ln(1/\alpha))$ vs $ln(t)$ is almost linear (with the exception of the initial value after 5 h ageing) displaying a good linear correlation coefficient of 0.99. The mean quadratic error between the values obtained with the single MAJ model and the experimental measurements on the annealed beads is estimated at 0.6. In comparison, this value is equal to 6.6 % for the new beads at 134°C. The n parameter for annealed beads is estimated to 1.8. As a reminder, the correlation coefficient of the single MAJ linear regression for as-received beads was of 0.93.

Residual stresses maps derived from Raman analyses, are shown in Fig. 6 with 146 cm⁻¹ as the reference position for the tetragonal peak.

These values revealed the existence of local compressive stress zones (shown red/orange in Fig. 6) on the new beads, where values up to −0.5 GPa were reached. In contrast, more homogeneous stress-maps were obtained on thermally treated samples (stress-annealed) with only few compressive stress zones reaching −0.2 GPa.

### 3.3. Effect of the ageing medium

Since ageing in the digestion vessels occurs with the beads fully immersed in the liquid, whereas ageing in autoclave typically involves specimens in contact with the steam, the first step was to determine
<figure><img src="image_5.png" /><figcaption>**Fig. 3.** (Left) Fitting of ln(ln(1/1-α))) versus ln(t) for Zirmil-Y ageing values. Blue dot lines represent the linear fitting with single MAJ law and solid red lines represents the double MAJ. (right) Linear fitting of ln(b) as a function of 1/T for the single MAJ and the two parts of the double MAJ.</figcaption></figure>

<table><caption>**Table 3**

Parameters of the double MAJ law and the activation energies associated.</caption><thead><tr><th>Solution</th><th>n</th><th>V<sub>m,saturation</sub></th><th>Activation energy (kJ/mol)</th><th>b<sub>0</sub></th></tr></thead><tbody><tr><td>1st part</td><td>n<sub>1</sub> = 1.2</td><td>V<sub>m1</sub> = 8.5 %</td><td>E<sub>1</sub> = 110</td><td>b<sub>0, 1</sub> = 6.2 E + 14</td></tr><tr><td>2nd part</td><td>n<sub>2</sub> = 3.9</td><td>V<sub>m2</sub> = 61.7 %</td><td>E<sub>2</sub> = 129</td><td>b<sub>0, 2</sub> = 1.4 E + 13</td></tr></tbody></table>

whether ageing in liquid water is equivalent to ageing in water steam (at identical temperature and pressure). To investigate this, new Zirmil-Y beads were subjected to the same autoclave run (134°C, 100 h) while immersed in water or positioned above a grid in the upper part of the autoclave in contact with steam. After this ageing treatment, the transformed monoclinic phase volume fraction was 57 % for water and 53 % for water steam, showing that the two types of procedures give similar results in terms of ageing and that it should therefore be possible to compare the ageing behaviour obtained in water using the digestion vessels with that observed on water steam in the autoclave. Fig. 7 illustrates that a time lag exists between the ageing treatment carried out in the vessel and in the autoclave, with the curve from in the vessel is clearly shifted to longer ageing times. In both ageing procedures, treatment sequences of 5 h at 134 °C were applied. However, in the case of autoclave ageing tests, the 5 h timer begin once the temperature reaches 134°C. Conversely, for digestion vessel ageing, the timer starts as soon as the vessel is placed in the oven. Therefore, the effective time of ageing at 134°C in vessels is shorter due to the time required to heat

the different layers (steel, PTFE and solution) of the digestion vessel. By superimposing the two curves, it can be estimated that the solution in the vessel spent approximately 3 h and 36 min at 134 °C rather than the 5 h of treatment in the oven. The heating time required for the solutions in the digestion vessels to reach 134°C is estimated to be around 1 h and 24 min. The corrected ageing behaviour in the vessel is also presented in Fig. 7 (red curve), where the step between each point is set at 3 h 36 min (referred as effective ageing time) instead of 5 h.

In the following analysis, the same time correction due to the heating time of the vessel was applied for all curves and the new name of the abscissa changed to “effective ageing time”. The experimental results of ageing tests conducted in vessels at 134°C under acidic, neutral and basic conditions for different effective ageing times are shown in Fig. 8.

To evaluate the repeatability of ageing results, the standard deviation of the monoclinic volume fraction was estimated from four ageing tests performed at the same conditions (i.e. pH=11 with soda, on new Zirmil-Y beads at 134°C for 75 h), and was evaluated at ± 1.7 vol%. For a given effective ageing time, the differences between the ageing

<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Observation of the polished cross-section of aged beads, 60 h at 134 °C. The boundary of the transformed zone is highlighted by a red solid line. Zoom in on a crack propagating below and parallel to the surface in the transformed zone.</figcaption></figure>
<figure><img src="image_7.png" /><figcaption>Fig. 5. (Left) Ageing curves at 134 °C of new and annealed beads. (Right) Fitting of ln(ln(Vmax/(Vmax-Vm))) versus ln(t) for stress-annealed bead: blue.</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 6. Stress maps (from the shift of the 146 cm⁻¹ tetragonal band) collected at the surface of Zirmil-Y beads: As-received (Left) annealed (Right).</figcaption></figure>

kinetics observed in different media in Fig. 8 can be considered significant and not attributable to variability.

As shown in Fig. 8, the evolution of the monoclinic phase content occurs in two steps for all media. Slow and similar t-m transformation kinetics are observed until almost 15 vol% then a slope inflection is reached, and the t-m transformation accelerates (stage II described above). The slope inflection isn't reached at the same time in all the solutions: it takes 65 h in soda solution to reach it, versus 75 h in water and ammonia and 80 h in nitric acid (Table 4). Therefore, the acceleration of the transformation depends on the media, NaOH leading to an acceleration of ageing before the one observed in water, while HNO₃ delays this phenomenon. It is also interesting to note that not only the pH plays on this onset of ageing acceleration, since for the same pH value of 11, results are slightly, but significantly, different between NaOH and NH₃.

## 4. Effect of the pH of the ageing medium

Two other ageing tests at 134 °C in vessels were studied at lower and higher pH using solutions of HNO₃ at pH= 2 and of NaOH at pH= 12 (Fig. 9, empty symbols). For comparison, the ageing results obtained at pH= 3 (HNO₃) and pH= 11 (NaOH) were added to this figure (full symbols).

In the case of NaOH solutions, the ageing behaviour is significantly affected by increasing the pH from 11 to 12, as LTD is shifted to higher effective ageing time (for example, 40 vol% of monoclinic phase was

obtained after 68 h of ageing at pH=11 whereas 80 h were necessary to reach the same monoclinic transformation at pH=12). The increase of the pH from 11 to 12 has therefore slowed down the ageing of Zirmil-Y beads. In contrast with this result, decreasing the pH of the solution from 3 to 2 seems to have little effect on the ageing of the Zirmil-Y beads.

The average pH difference of acid and basic solutions before and after each ageing sequence of 5 h in vessels is presented in Table 5.

For solutions at pH= 11, an average of the pH difference of -0.11 and -0.31 every 5 h is observed in the case ammonia and of sodium hydroxide, respectively. The nitric acid solution did not show any significant pH changes during the ageing tests. To determine if the observed pH changes result from a possible chemical interaction between the beads and the solution during ageing, the same ageing procedure was conducted in NaOH solution at pH= 11 but without adding beads in the digestion vessel; the same solution was kept at room temperature in a closed digestion vessel as a control. After 5 h at room temperature, the pH decreased to 10.85 (i.e. ΔpH= -0.15), and at 134 °C, it dropped to 10.75 (i.e. ΔpH= -0.25). This result indicates that the pH decrease cannot be clearly attributed to the presence of the beads.

The ICP results after two long ageing treatments of beads, performed at 134 °C for 40 h and 75 h and without solutions renewal (i.e. in one-step treatment), are given in Table 6. Decrease in pH similar to those reported in Table 5, were also observed during these long tests.

Al³⁺ ions are scarcely detected in solutions after ageing in water or nitric acid at pH= 3. However, decreasing the pH of the nitric acid
<figure><img src="image_9.png" /><figcaption>**Fig. 7.** Comparison of the ageing behaviour of new Zirmil-Y beads with two different devices at 134 °C: autoclave (water steam) and digestion vessel (immerged into water). The corrected digestion vessel results, taking into account a heating time difference of 1 h and 24 min per step, are plotted in red.</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 8.** Monoclinic volume fraction ($V_m$) on new Zirmil-Y beads versus effective ageing time at 134°C in different solutions (nitric acid, water, soda and ammonia).</figcaption></figure>

<table><caption>**Table 4**

Time to reach the slope inflection in each solution.</caption><thead><tr><th>Solution</th><th>HNO<sub>3</sub><br>pH=3</th><th>H<sub>2</sub>O</th><th>NaOH<br>pH=11</th><th>NH<sub>3</sub><br>pH=11</th></tr></thead><tbody><tr><td>Time to reach the slope inflection</td><td>80 h</td><td>75 h</td><td>65 h</td><td>75 h</td></tr></tbody></table>

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Ageing curves in NaOH solution at pH= 11 and 12 (yellow), in HNO₃ at pH= 2 and 3 and in water as a function of the real ageing time.</figcaption></figure>

<table><caption>**Table 5**

Average pH difference for a sequence of 5 h in different solutions during ageing tests in vessels.</caption><thead><tr><th></th><th>HNO<sub>3</sub><br>pH=2</th><th>HNO<sub>3</sub><br>pH=3</th><th>NH<sub>3</sub><br>pH=11</th><th>NaOH<br>pH=11</th><th>NaOH<br>pH=12</th></tr></thead><tbody><tr><td>ΔpH<sub>average</sub></td><td>- 0.11</td><td>- 0.01</td><td>- 0.11</td><td>- 0.31</td><td>- 0.17</td></tr></tbody></table>
<table><caption>**Table 6**
ICP analysis of Al³⁺, Y³⁺ and Zr⁴⁺ content in solution after one-step long time ageing at 134°C for sequences of 40 h and 75 h.</caption><thead><tr><th>Element</th><th>Single step sequence Time (h)</th><th>HNO<sub>3</sub><br>pH= 2<br>(ppm)</th><th>HNO<sub>3</sub><br>pH= 3<br>(ppm)</th><th>H<sub>2</sub>O<br>(ppm)</th><th>NH<sub>3</sub><br>pH= 11<br>(ppm)</th><th>NaOH<br>pH= 11<br>(ppm)</th><th>NaOH<br>pH= 12<br>(ppm)</th></tr></thead><tbody><tr><td rowspan="2">Al<sup>3+</sup></td><td>40</td><td>n.a</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>1.26</td><td>2.54</td><td>n.a</td></tr><tr><td>75</td><td>1.36</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>4.44</td><td>9.40</td><td>6.00</td></tr><tr><td rowspan="2">Y<sup>3+</sup></td><td>40</td><td>n.a</td><td>2.64</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>n.a</td></tr><tr><td>75</td><td>5.54</td><td>2.88</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 1*</td></tr><tr><td rowspan="2">Zr<sup>4+</sup></td><td>40</td><td>n.a</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>n.a</td></tr><tr><td>75</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 0.05</td><td>&lt; 1*</td></tr></tbody></table>

n.a: not analysed. *In sodium hydroxide at pH= 12, a 20-fold dilution with HCl solution was performed in order to be in acid conditions for ICP analysis, thus the precision of measure is reduced at < 1 ppm.

solution to pH= 2 leads to the dissolution of a small amount of Al₂O₃. In basic solutions, Al³⁺ ions are detected, and their content appears to increase with ageing time. Furthermore, NaOH seems to dissolve more Al₂O₃ than NH₃ solution. Surprisingly, the content of Al³⁺ ions decreases when the pH increases from 11 to 12 in NaOH. Regarding Y³⁺ ions, they are almost not detected in basic solutions. However, their concentration increases in nitric acid solutions with the ageing time and the pH decrease. No significant concentration of Zr⁴⁺ ions was detected in all the tested solutions.

# 5. Discussion

## 5.1. Double-MAJ ageing kinetics

The ageing tests in autoclave at various temperatures (Fig. 3) revealed that new Zirmil-Y beads did not age following the commonly observed single MAJ law as typically reported in the literature [4,6,8,18, 27]. In these studies, the application of MAJ theory on monoclinic content evolution during ageing of Y-TZP fitted very well with the experimental results with a sigmoidal shape curve. It also gives access to the activation energy Q of this transformation and the “n” parameter describing the main nucleation-growth mechanism occurring during this transformation.

However, in this study the ageing of the Y-TZP beads occurred in two stages: initially, with relatively slow kinetics until reaching 15 vol% of the monoclinic phase, followed by an acceleration of the t-m transformation kinetics. This leads to curves of the monoclinic volume fraction as a function of ageing time which increase to an intermediate plateau and then rise again until the second plateau related to the monoclinic saturation (Fig. 2).

The monoclinic phase appears at the surface and then grows towards the core of the beads (Fig. 2) and monoclinic content was evaluated by XRD. It is thus interesting to estimate the thickness of the monoclinic phase layer corresponding to 15 vol% analysed by XRD. According to the literature, 90 % of the signal analysed by XRD comes from the 5 µm surface layer of a Y-TZP sample [21,22]. To estimate the monoclinic layer thickness during ageing from XRD results, the transformed layer can be modeled as a spherical shell with thickness (t). The volume of this transformed shell (V<sub>mono</sub>) was calculated, along with the volume of a spherical shell with a thickness of 5 µm (V<sub>DRX</sub>). The ratio of V<sub>mono</sub>(t) to V<sub>DRX</sub> provides an approximation of the volumetric percentage of the transformed phase detected by XRD in zirconia. Taking this into account, it can be estimated that the slope inflection at 15 vol% (Fig. 2) occurs when a continuous and fully transformed 700 nm thick monoclinic layer is reached, which represents more or less the thickness of few zirconia grains.

An ageing model based on experimental results is developed in this study using a double MAJ law (see Eq. 4), which gives a good fit between the experimental and calculated data. The double MAJ model describes both the slow ageing stage and the acceleration observed beyond the slope inflection, as shown in Fig. 3. This model enables the estimation of the activation energies for the two parts of the t-m transformation at

E₁= 110 and E₂= 130 kJ/mol, as detailed in Table 3. The values obtained are of the same order of magnitude as those in the literature [4,8] where Q typical values are between 90 and 115 kJ/mol. Each part of the double MAJ law has a different n parameter value. The first part is described by n₁= 1.2, which can be related to instantaneous nucleation and one-dimensional growth, and the 2nd part has n₂= 3.9, related to sporadic nucleation and three-dimensional growth mechanisms [7,28] if interface-controlled growth is assumed. This behaviour could indicate the existence of two distinct tetragonal Y-TZP grain populations from the surface of the beads, each behaving differently during ageing. The compressive stresses at the outer surface delimit the first population of grains and could also be responsible of the needle-like transformation of m-grains at the beginning of ageing, as suggested by the small value of n₁, instead of fully transformed m-zone that could grow in the second stage of the curve, as suggested by the higher value of n₂ [29]. Our results suggest that an initial population of tetragonal grains rapidly transforms by instantaneous nucleation from the beginning of the ageing process, reaching a plateau at about 9 vol% (as shown in Fig. 10). This "population" would be located on the extreme surface or sub-surface of the beads. Then, a second population, deeper from the surface, transforms following a more classical kinetics, with a pronounced sigmoid shape of the curve (second part of the double MAJ law). This second part corresponds to approximately 62 % of the transformed monoclinic fraction measured by XRD.

The ageing tests conducted on annealed beads, as shown in Fig. 5, reveal that the observed ageing delay, before reaching 15 vol% of monoclinic amount, disappears after a thermal treatment at relatively low temperature and short time (i.e. 1200°C for 12 min). Furthermore, Raman analyses indicate the presence of local compressive stresses on the surface of new Zirmil-Y beads, estimated at maximum -0.5 GPa, which significantly decrease (> -0.2 GPa) after beads were thermally treated. In as-received beads, the relatively high residual stresses may be attributed to the polishing post-process applied. Therefore, it is feasible to correlate the delay in ageing to the residual compressive stresses present on the as-received beads, a study from Deville et al. also revealed that compressive stresses, arising from a “rough polishing”, could improve the ageing resistance of Y-TZP [29]. The same protective effect against the ageing of Y-TZP due to residual compressive stresses was also observed in the study of Guo et al. [30]. In the same manner, another study from Wei et al. [31] has also concluded that by removing tensile residual stresses, ageing was slowed down by eliminating potential nucleation sites. This behaviour complements the findings of our study where compressive stresses decelerate the t-m transformation.

In a previous work, Gremillard et al. [7] showed that the “n” parameter could evolve during ageing due to the concomitance of nucleation and growth process during LTD of Y-TZP. It is shown in Fig. 11, derived from the curves Fig. 5, that in the case of annealed beads, this parameter is almost constant compared with as-received beads where n evolve from 0.5 to 4.2 with a transition after 45 h of ageing. This is correlated with the slope inflection time observed at 45 h at 134°C and around 15 vol% on as-received beads, showing that a transition has occurred in the nucleation growth process.
<figure><img src="image_12.png" /><figcaption>Fig. 10. Decomposition of the double MAJ law at 134°C in autoclave.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>Fig. 11. Evolution of n parameter, derived from the slope of the ln(ln(Vmax/Vmax-Vm)) in function of ln(t) during ageing tests on as-received beads and annealed beads at 134°C.</figcaption></figure>

During the first part of the ageing of Zirmil-Y beads, the double-MAJ law gives a n₁= 1.2 which corresponds mainly to nucleation process and is in good agreement with the n parameter obtained by differentiation of the curve at 134°C presented in Fig. 5. The two values are different because the model gives a mean value of n during the first part of the ageing contrary to the differentiation of the curve which gives an “instantaneous” n parameter. The same is observed with the second part of the double MAJ, n₂= 3.9 which is not exactly equal to the “instantaneous” n parameter. The fall at the end of the curve was also observed in the study of Gremillard et al. [7] with other 3Y-TZP and can be attributed to the fact that the Vm plateau has been reached.

## 5.2. Effect of the ageing medium

Traditionally, ageing studies on Y-TZP have been conducted in an

autoclave using water steam [6,8,18,27]. However, this method cannot be used to test the effects of different pH levels and different types of solution on ageing, unlike the use of digestion vessels employed in this study. Fig. 7 demonstrates that treatment in a digestion vessel is comparable to a treatment in an autoclave, showing that Y-TZP beads aged similarly in water steam and when immersed in water, as demonstrated in this study.

In a previous study, Sato had conducted ageing tests in sealed stainless steel tubes with various acids, bases, and solvents [18]. However, this approach did not enable the kinetic of ageing of the Y-TZP in different solutions to be assessed. The results presented in Fig. 8 indicate that differences do exist between ageing kinetics measured in the different solutions and depend on the chemical species in solution.

The ageing mechanism commonly accepted is due to the diffusion of OH⁻ ions from water into Y-TZP via oxygen vacancies [11]. The diffusion kinetics of OH⁻ are therefore linked to its chemical activity. The chemical activity of OH⁻ depends not only on the pH but also on the ionic strength of the solution, the presence of other species, and how these species interact. Ionic strengths are the same, since the OH⁻ concentration is the same, as are NH₄⁺ and Na⁺. Most of the NH₃ remains undissociated, as ammonia is a weak base, and can then interact with OH⁻ via a weak hydrogen bond that reduces the “free” OH⁻ ions in the solution, thus decreasing its chemical activity. As a result, the chemical activity of OH⁻ is lower in an ammonia solution than in a soda solution, despite identical OH⁻ concentrations in both solutions.

Furthermore, the ICP results, Table 6, obtained after two prolonged ageing periods, 40 h and 75 h, revealed that in the case of NaOH and NH₃, Al₂O₃ in the beads was dissolved in the solutions in the form of Al³⁺. This dissolution was not observed in HNO₃ solutions except at very acidic pH where a small amount of Al³⁺ is detected. Indeed, alumina exhibits good corrosion resistance [32] in a nitric acid environment, but not in a basic one [33]. The dissolution of alumina, stronger for soda than for ammonia at pH= 11, could be correlated to the higher OH⁻ chemical activity which has induced the faster ageing kinetics observed with soda.

Adjusting the pH of a solution may or may not affect ageing. Fig. 9 illustrates that in the case of nitric acid, reducing the pH from 3 to 2 does not result in a modification in the ageing behaviour of the beads. Yet, there are fewer OH⁻ ions in the solution. On the other hand, increasing the pH in the NaOH solution leads to a substantial change in the behaviour of the beads, with a pronounced drop of the ageing kinetics
observed in the highly basic environment at a pH= 12. This behaviour is surprising as there should be more OH⁻ ions present. However, this is correlated with a diminution of the Al³⁺ concentration in the suspension at pH= 12 in NaOH compared to pH= 11 (Table 6). Several studies were already performed on the effect of alumina addition to reduce the t-m transformation of Y-TZP during ageing [34–36]. These studies proposed two different hypotheses to explain this stabilization through Al₂O₃ addition. Li et al. [35] explain the increase of ageing resistance through a reaction between Al₂O₃ particles with H₂O resulting in the formation of a Al(OH)₃ at the surface of the Y-TZP acting like a protection against ageing. The other explanation is the segregation of Al₂O₃ at the grain boundary which could increase the cohesion of the grains preventing the generation of cracks during transformation and thus diffusion of OH⁻ ions in the bulk of Y-TZP. In our study, the dissolution of alumina could lead to a better diffusion of OH⁻ ions through grain boundaries and then lead to an acceleration of the transformation of first surface layers in contact with the solution. In the case of NaOH solution, at pH= 11, higher dissolution of alumina was detected compared with pH= 12, with consequently an acceleration of the ageing at pH= 11. However, this is an observed phenomenon which is a consequence and does not explain the origin of this behaviour. One explanation may lie in the fact that at basic pH, precipitation of Al(OH)₃ occurs, but as the pH continues to rise (in our case from 11 to 12), more Al(OH)₃ dissolves into Al(OH)₄. The higher concentration of Al(OH)₄ at pH= 12 than at pH= 11 increases the ionic strength of the solution close to the beads, reducing the chemical activity of OH⁻ and thus ageing.

On the other hand, yttrium, used as a stabilizer of the tetragonal phase of zirconia, is dissolved during ageing in the presence of nitric acid but not in the presence of the two bases, NaOH and NH₃. This phenomenon does not seems to be correlated to the ageing kinetics and had already been observed in a study on Y-TZP with a linear dependence of the amount of yttrium released with a decrease in pH [37].

In the light of these ageing results, coupled with chemical analysis outcomes, it is conceivable that alumina leaching plays a more significant role in low-temperature degradation (LTD) of Zirmil-Y beads than yttrium [38]. Indeed, alumina, used as a sintering agent, is mainly located at the grain boundaries [39]. During ageing tests in digestion vessels these grain boundaries can be dissolved, allowing better penetration of OH⁻ ions responsible for zirconia ageing. The quantities of yttrium and aluminium released during the ageing tests are consistent with a chemical modification of the material at a thickness of about one grain (around 350 nm). An estimation of the final composition of the surface grains is given in supplementary information.

## 6. Conclusion

This study enhances our understanding of the ageing behaviour of 3Y-TZP grinding media, particularly those used in wet milling. The polishing step applied on Zirmil-Y beads induces compressive surface stresses, which temporarily slows down the LTD kinetics. The ageing kinetics can then be effectively described using a double MAJ law, which illustrates the two stages of the mechanism: first, sporadic ageing at the surface, followed propagation to the bulk. This highlights the critical role of surface finishing on the ageing resistance of Y-TZP pieces.

The newly developed accelerated ageing protocol using digestion vessels has yielded results comparable in water to those obtained with autoclave treatment. Ageing in digestion vessels has thus allowed the observation of the effects of the environment in which the Y-TZP beads are aged, which was previously impossible with autoclave treatments. This revealed that the ageing mechanism of Y-TZP is influenced both by the chemical species in which the ageing occurs and by the pH of the solution. It appears that NaOH solution accelerates ageing through the leaching of Al³⁺ ions, unlike nitric acid solution, which seems to slow down the ageing of the beads, even if Y⁴⁺ ions dissolution is observed.

This study could provide insights into the ageing of 3Y-TZP when exposed to various acidic and basic environments in real-life

applications (dental restorations, implants, ceramic grinding media, etc). However, it is important to highlight that the present study only evaluates the material behavior, without taking into account other factors that may also contribute to ageing, such as mechanical loading, temperature and the chemical environment. Further research is still needed to better understand the ageing of zirconia subjected to several simultaneous stresses (i.e. mechanical, thermal, chemical...) and their possible coupling effects on the kinetics of ageing.

## Declaration of Competing Interest

This study was partially financed by Saint-Gobain Research Provence and ZirPro (France).

## Acknowledgements

Authors thank B. Reynard and G. Montagnac for assistance during the Raman experiments at LGL-TPE ENS Lyon. Thanks are due to the CLyM (Consortium Lyon Saint- Etienne de Microscopie: www.clym.fr) for the access to the microscope SEM SUPRA ZEISS. CLyM is supported by the CNRS, the "Grand Lyon" and the "Rhône-Alpes Region". The first author receives financial support by ANRT grant number CIFRE 2022/0301, Saint-Gobain Research Provence (SGR Provence) and ZirPro.

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