# Temporal evolution of the morphology of contaminants deposits

Aurélie Zamo, David Lansade, Jean-François Roussel, Guillaume Rioland, Delphine Faye

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Aurélie Zamo, David Lansade, Jean-François Roussel, Guillaume Rioland, Delphine Faye. Temporal evolution of the morphology of contaminants deposits. IOP Conference Series: Materials Science and Engineering, 2025, 1328, 10.1088/1757-899x/1328/1/012016 . hal-05178359

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Submitted on 23 Jul 2025

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# Temporal evolution of the morphology of contaminants deposits

Aurélie Zamo<fnref n="1" />, David Lansade<fnref n="1" />*, Jean-François Roussel<fnref n="1" />*, Guillaume Rioland<fnref n="2" /> and Delphine Faye<fnref n="2" />

*E-mail: david.lansade@onera.fr / jean-francois.roussel@onera.fr

**Abstract.** The effects of contamination on space systems have been studied for many years. However, the effects especially due to the morphology of contaminant deposits have not been studied extensively. ONERA has developed a vacuum chamber for studying molecular contamination in which experiments aimed at studying changes in morphology. Ex-situ optical microscopy analyses revealed temporal maturation phenomena: coalescence and solidification of the deposit. A systematic study of contaminant/substrate/pressure conditions combinations allowed a first understanding of the influence of the pressure on changes in deposit morphology after contamination. These results show the importance of identifying the morphology of contaminant deposits within a short delay in order to anticipate changes in morphology.

## 1. Introduction

Exposed to space environment, some materials that make up satellites outgas molecules, known as contaminants. These contaminants can condense on sensitive surfaces of the satellite, significantly altering their properties. The main consequences of this contamination are loss of transmittance in optics [1], reduced solar panel power output [2], and changes in solar absorptance of thermal coatings [3].

In-flight measurements showing differences in aging behaviour of various contaminated materials were previously poorly understood. Recent work suggests differences in morphology (cluster vs. film) of the contaminant deposits may be responsible for these effects. The way the contaminant deposits age under ultraviolet (UV) irradiation is greatly influenced by their morphology (film vs clusters). For example, the increase in solar absorptivity of a contaminated substrate can be highly important in the case of a film deposit. Despite the interest in contamination morphology [4], the physical processes are little studied and, moreover, difficult to predict.

One issue for studying the morphology of contaminant deposits when there is no in-situ measurement possible is the need to restore atmospheric pressure to characterize the samples. The experiments presented in this paper aim at determining the influence of the pressure (vacuum breakdown) on potential changes in deposit morphology after contamination.

In order to establish the effects of atmospheric pressure on contaminated surfaces, a study was carried out comparing the pressure conditions under which the samples were held. To achieve this, a systematic study of contaminant/substrate/deposition temperature combinations and characterizations of contaminated surfaces are used to understand the phenomena governing the morphology of a deposit, thus refining predictions.

<figure><img src="image_2.png" /></figure>

<footnote n="1">¹ DPHY, ONERA, Université de Toulouse, Toulouse, France</footnote>

<footnote n="2">² CNES, Toulouse, France</footnote>
## 2. Experimental

### 2.1 Vacuum contamination chamber

ONERA has developed a vacuum test facility for studying contamination mechanisms. The facility is equipped with an effusion cell in which a certain quantity of contaminant is introduced. Once the effusion cell reaches a specific temperature, a gaseous flow of contaminant is generated. The presence of a temperature-regulated copper envelope (known as shroud) enhances the transport of molecules (contaminant gas flow) towards various targets: substrates and quartz crystal microbalances (QCMs). These targets, positioned on a sample holder, can also be temperature-controlled to study contaminant deposition according to various parameters.

QCMs enable real-time monitoring of contaminant deposition and detection of any reemission of deposits. QCMs used in this work are equipped with quartz crystals vibrating at a resonant frequency of 6 MHz. The crystal is a quartz disk 14 mm in diameter, with a gold coating on both electrodes faces. The quartz crystals used are “AT-cut” and have been cut according to a particular crystallographic direction (Neyco® AUDA 6).

Figure 1 shows the positioning of the various elements described above.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** COPHOS test equipment (Credit: ONERA).</figcaption></figure>

### 2.2. Contaminants and materials

When a material outgases, a multitude of contaminants are emitted and can contaminate a satellite surface [5]. Given the diversity of outgassing species, it is complex to determine the impact of each species on the morphology of the deposit. We have therefore focused on studying
the morphology of the deposits of a pure compound: diphenyl phthalate. This contaminant is representative of molecules outgassed in flight as it belongs to a group of molecules often found as plasticizers. Phthalates can be found from polymers such as polyesters and polyurethanes. These polymers are used in adhesive tapes, resins or thermal control coatings (MLI). Contaminated substrates are materials that can be used for optical surfaces, thermal control or solar panels, like: fused quartz, polyimide (Kapton tape), black silicone paint.

## 2.3. Characterization of contaminant deposits: optical microscopy

Deposits need to be accurately characterized in order to understand their behavior on various substrates and consequently determine the physico-chemical mechanisms responsible for changes in morphology. Thus, various characterization techniques have been employed.

To characterize the deposits, an optical microscope (Nikon LV 100 Eclipse) coupled to a camera (Imaging Source - DKF330X365) was used. By modulating the contrast or the angle of inclination of the light, it is possible to observe micrometric deposits. This makes it possible to determine the morphology of deposits (film or droplets), droplet diameters and sphericity. The morphology of the studied deposits can also be observed at different times.

# 3. Results

## 3.1. Test plan

Samples were contaminated with diphenyl phthalate. QCMs and samples surfaces were kept at a constant temperature of 5 °C for the whole deposition time under secondary vacuum ($P \sim 1 \times 10^{-6}$ mbar). Such temperature inhibits contamination reemission under vacuum, detrimental to the study. When the QCMs and samples holder are stable, the effusion cell is heated and maintained at 190 °C. The temperature of the effusion cell is relatively high, which is unusual for spacecraft. This choice of temperature was motivated by the objective of minimizing the time required for the deposit. As soon as the effusion cell is empty (no more incident molecular flow), the pressure in COPHOS is:

- Case 1: maintained for 3 days under dry nitrogen at 1 bar

- Case 2: maintained for 3 days under secondary vacuum ($P \sim 1 \times 10^{-6}$ mbar)

The mass of diphenyl phthalate in the effusion cell is 10.2 mg and 10.5 mg for case 1 (dry nitrogen) and case 2 (secondary vacuum), respectively.

## 3.2. QCM measurement of deposit mass

Despite targets' temperature maintained at 5 °C, a reemission phenomenon is detected in the case of a deposit maintained under secondary vacuum, whereas it does not appear under dry nitrogen at 1 bar (Figure 2). This difference is explained by the fact that when maintained under secondary vacuum, the reemission flow from the targets surface is in free molecular regime. After reemission none of the molecules collide, so no return to the contaminated surface is possible. On the opposite, a flow of reemitted molecules under dry nitrogen has a high probability of colliding with nitrogen molecules close to the contaminated surface due to the fact that the mean free path at such pressure is in the order of 100 nm [6], [7]. As a result, almost every reemitted molecule return to the surface and the reemitted flux becomes so weak that it is not measurable on QCMs.
<figure><img src="image_4.png" /><figcaption>**Figure 2.** Temporal evolution of Knudsen cell and shroud temperatures for a) case 1 maintained under dry nitrogen at 1 bar and c) case 2 maintained under secondary vacuum at 1x10⁻⁶ mbar.</figcaption></figure>

Temporal evolution of diphenyl phthalate deposited and re-emitted masses measured on QCM à 25 °C for b) case 1 maintained under dry nitrogen at 1 bar and d) case 2 maintained under secondary vacuum at 1x10⁻⁶ mbar (Credit: ONERA).

*Note: QCM 1 malfunctioned during the experiment. Due to the geometry of the contamination system, the deposited mass is assumed to be close to that measured on QCM 4.*

### 3.3. Morphology of contaminant deposits

It is important to note that the contaminated samples presented below were maintained for 3 days under dry nitrogen at 1 bar or under secondary vacuum before being characterized by optical microscopy (at ambient pressure and room temperature). It was possible to observe different morphologies depending on the nature of the contaminated substrate, regardless of the pressure conditions in the chamber (Figures 3 to 5). Observations of every samples as a function of time were also carried out in order to consider possible changes in morphology.

#### 3.3.1. Polyimide

At first sight, it seems difficult to detect any major differences between contaminated polyimide samples maintained under dry nitrogen or secondary vacuum: the deposits form droplets (Figure 3). However, the surface area covered by diphenyl phthalate molecules appears to be greater in the case of deposits maintained under dry nitrogen at 1 bar. This phenomenon results from the reemission of deposit under secondary vacuum. Smallest droplets evaporate more easily from the surface and cannot return to the surface (free molecular flow).

In the case of contaminated samples maintained under dry nitrogen, numerous droplets are in contact with each other, but it is still possible to observe isolated droplets. For the most part, the droplets show an approximately central zone with no apparent deposit. The droplets are between 20 and 30 micrometers in size. Deposition on polyimide maintained under secondary
vacuum for 3 days also shows droplets between 16 and 25 micrometers, with a central zone without apparent substance.

Temporal analysis of the samples was carried out 1 month after they were exposed to air (samples are stored in closed boxes at 20% humidity and 22°C.). The results of these observations confirm changes in morphology. However, these temporal maturation phenomena are not identical between the two experimental conditions. In the polyimide sample maintained under dry nitrogen for 3 days, droplets solidification appears, with a reduction in size (ten micrometers) and also the formation of dendrites. In the experiment maintained under secondary vacuum, the clusters also solidify and the smallest clusters seem to have evaporated.

<figure><img src="image_5.png" /><figcaption>**Figure 3.** Optical microscopy imaging of the temporal evolution of diphenyl phthalate deposits on polyimide samples for different pressure conditions (Credit: ONERA).</figcaption></figure>
### 3.3.2. Fused quartz

Figure 4 shows that diphenyl phthalate is deposited on a fused quartz surface in the form of droplets (maintained under secondary vacuum and dry nitrogen).

Although the deposit is spread over a large part of the sample surface, in the case of the deposit maintained under dry nitrogen, the larger droplets are surrounded by smaller droplets (~5 µm). Large areas without apparent deposit structure are also visible over a large part of the surface. In the case of deposits maintained under secondary vacuum for 3 days, the droplets show a less well-structured external structure than in the previous case. The number of small droplets also appears to be reduced compared with the case maintained under dry nitrogen at 1 bar. The absence of smaller droplets can be explained by the reemission of the deposit under secondary vacuum. As the ratio surface/volume is higher for small structures, their evaporation from the surface is accentuated.

Observation of the fused quartz samples 1 month after opening COPHOS reveals that maturation phenomena are visible at ambient pressure and room temperature. It is possible to distinguish the effects of coalescence between droplets, crystallization and dendritic growth. However, these phenomena only seem to be visible when the sample was kept under dry nitrogen at 1 bar for 3 days. In fact, when the sample was kept under secondary vacuum for 3 days before being returned to air, the coalescence process seemed weaker. This slower maturation may be due to the greater distance between droplets, which prevents them from interacting with each other.
<figure><img src="image_6.png" /><figcaption>**Figure 4.** Optical microscopy imaging of the temporal evolution of diphenyl phthalate deposits on fused quartz samples for different pressure conditions (Credit: ONERA).</figcaption></figure>

### 3.3.3. MAP PNC, silicone-based black paint

No matter what pressure conditions were applied to the silicone-based black paint samples (kept under 1 bar dry nitrogen or under secondary vacuum), surface coverage by the diphenyl phthalate deposit was heterogeneous. Figure 5 shows a few areas presenting diphenyl phthalate crystals. However, a difference in crystal shape is visible 1 month after the samples were re-exposed to the air. In the case where the deposit was kept under dry nitrogen for 3 days, the crystals are concentrated together. On the contrary, when maintained under secondary vacuum for 3 days, the deposits observed is in the form of needles, when it initially appeared to be in the form of small clusters.
The composite nature of this silicone-based black paint and its roughness probably influence strongly the adsorption process of molecules on the surface.

MAP PNC, silicone-based black paint

<figure><img src="image_7.png" /><figcaption>**Figure 5.** Optical microscopy imaging of the temporal evolution of diphenyl phthalate deposits on silicone-based black paint samples for different pressure conditions (Credit: ONERA).</figcaption></figure>

**4. Conclusion**

The effects of pressure on morphological changes in diphenyl phthalate deposits were investigated. For this purpose, diphenyl phthalate deposits were studied on various substrates (fused quartz, polyimide and silicone-based black paint) with a deposit maintained under dry nitrogen at 1 bar and another one under secondary vacuum.
When contaminants are deposited in the gas phase, some of the molecules adsorbed on a surface may reemit. However, this reemission flux can be modified according to the conditions under which the deposit is maintained. When the deposit is maintained under secondary vacuum, molecules are reemitted in free molecular flow and the associated flux is large. As a result, the smallest droplets evaporate and the quantity of molecules present on the surface lowers. When the deposit is kept under pressure at 1 bar, the molecular flux reemitted is drastically reduced as the reemitted molecules are subject to numerous collisions and may therefore return to the contaminated surface. These returns modify the morphology of the deposit: the smallest droplets are maintained on the contaminated surface and may interact one with each other.

The temporal analysis reveals a maturation process for all deposits and substrates. In-depth observations using optical microscopy (ex situ) revealed coalescence phenomena between several droplets and the transformation of droplets into crystals/dendrites. These phenomena, observed at ambient temperature and atmospheric pressure, occur as soon as the deposit is formed under secondary vacuum.

This study revealed that contaminant deposition morphology is dependent on a number of parameters: the nature of the contaminated surface, surface roughness and deposition pressure conditions. Further work is currently underway to determine the physico-chemical processes responsible for the morphology of contaminant deposits and to establish a means of in-situ characterization.

## References

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[2] Liu D., Liu S.H., Panetta C.J., Olson K.R., Hong S.M., Alaan D.R., Mann Colin J., and Luey K. 2010 Effects of contamination on solar cell coverglass 35th *IEEE Photovoltaic Specialists Conference* pp. 002563–002568.

[3] Remaury S., Nabarra P., Bellouard E., and d'Escrivan S. 2011 In-Flight Aging of Thermal Coatings: THERME Experiment *Journal of Spacecraft and Rockets* **48**(1) pp. 27-33.

[4] Luey K. T., Olson K. R., and Coleman D. J., 2018 Optical system contamination: formation of films and droplets *JATIS* **4**(3) p. 036001

[5] Lansade D., Roussel J.-F., Faye D., Rioland G., Suliga A., and Ergincan O., 2023 *IOP Conf. Ser.: Mater. Sci. Eng.*, vol. **1287**(1) p. 012022.

[6] Al-Dmour E. 2020 Fundamentals of Vacuum Physics and Technology 2017 *CERN-Accelerator-School course on Vacuum for Particle Accelerators*.

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