Patent Description:
Due to their mechanical properties and appealing aesthetics, ceramics and glass ceramics are widely used as dental materials. Today, ceramic materials are used in all types of indirect dental restorations: no-preparation veneers, thin veneers, bridges, multi-unit posterior fixed partial dentures (FPDs), etc..

One of the main advantages with glass ceramics is their translucency, which enables a high flexibility of the colour and the material can more easily be adapted the colour of the surrounding teeth.

Zirconia (ZrO<NUM>)-based ceramics are among the most well-studied dental materials due to their good mechanical properties, they are known as "ceramic steel", as well as being biocompatible. However, the high strength often comes with less good aesthetic properties, resulting in a more opaque material.

"<NPL> discloses glasses in the ZrO<NUM>-SiO<NUM> system containing up to <NUM> mol% ZrO<NUM>. The density, refractive index and hardness were all observed to increase with increasing ZrO<NUM> content.

<CIT> discloses a process for producing translucent ZrO<NUM>-SiO<NUM> nanocrystalline glass ceramic with ultra-high flexural strength by pressure-assisted sintering or pressure-less sintering.

Further Zirconia (ZrO<NUM>)-based ceramics are disclosed in <CIT>, <CIT>, <CIT> and in <NPL>.

In view of the prior art, there is a need for an enhanced glass ceramic composition with improved hardness, as well as pleasing aesthetic properties such as translucency and ability to resemble natural teeth, and a method for making such a composition, and a dental restorative material comprising such a composition.

The object of the present invention is to provide an improved glass ceramic material having a high strength and good translucency and a method of making such a material, this is achieved by the material in claim <NUM> and the method in claim <NUM>.

According to one aspect of the invention there is a glass ceramic material comprising zirconium dioxide crystals embedded in an amorphous silicon dioxide matrix and at least one hardness enhancing additive comprising aluminum oxide or yttrium oxide, or a combination of aluminum oxide and yttrium oxide in a concentration of <NUM>-<NUM> weight%. The zirconium dioxide crystals form cores and the cores are at least partly surrounded by rims, wherein the rims comprise an intergranular phase, and wherein the intergranular phase comprises at least silicon dioxide, zirconium dioxide and at least one hardness-enhancing additive. The concentration in weight percent of the hardness-enhancing additive is higher in the rims than in the amorphous silica matrix and in the cores.

According to one embodiment at least a portion of the cores are connected with at least one adjacent other core forming a grain boundary between the cores. The concentration of hardness-enhancing additive oxide is higher in the grain boundaries than in the parts of the rim in connection with the silicon dioxide matrix.

According to a second aspect of the invention there is a method of forming a glass ceramic material wherein the method comprises the following steps:.

wherein the method comprises a fractionation step reducing the particle size of the material.

According to a third aspect of the invention there is a densified material comprising a glass ceramic material.

According to a fourth aspect of the invention there is a dental restorative material comprising a densified material comprising a glass ceramic material.

In the following, the invention will be described in more detail, with examples and depending claims.

The terms 'nGC' or 'nGCs' is short for nanocrystalline glass ceramic(s), herein comprising ZrO<NUM> and SiO<NUM>;.

Glass ceramics of zirconia-silica (ZrO<NUM>-SiO<NUM>) comprises nano-sized ZrO<NUM> crystals embedded in an amorphous matrix of SiO<NUM>, such materials may be described as nanocrystalline glass ceramics (nGCs). Such materials are interesting for dental applications. However, there is a need for improvement of the hardness of such materials. Additionally, a dental material should preferably be optically translucent, since it will then appear more similar to natural teeth as compared to an opaque material, as well as being easier to color. Ideally, a dental material should be both hard and optically translucent.

Hardness is a mechanical property of the material, which can be tested in terms of microhardness and nanohardness. Microhardness is obtained by "microindentation hardness testing" and testing the hardness of the material on a microscopic scale. Nanohardness is obtained by "nanoindentation hardness testing", testing the hardness on a micro- or even nanometer scale using a very small tip size for the indentation object used. Other mechanical properties include Young's modulus and fracture toughness. Young's modulus measures the stiffness of a material, and defines the relationship between stress and strain. Fracture toughness is a measure of the stress required in a material for a crack to propagate rapidly. It is of interest to improve, i.e. increase the hardness (micro and nano) and possible also other important mechanical or optical properties of the materials. Biaxial strength is the stress at failure in bending, and represents the highest stress experienced within the material at its moment of yield.

In order to improve the hardness, while still keeping the translucency of a glass ceramic material comprising ZrO<NUM> crystals and amorphous SiO<NUM> at an acceptable level at least one hardness enhancing additive is added to the material. A majority of the ZrO<NUM> cystals are connected with adjacent crystals forming grain boundaries (GBs) in at least one direction.

The above described microstructure is schematically illustrated in <FIG> a and b, wherein cores <NUM> are surrounded by rim <NUM>, the cores <NUM> with the rims <NUM> arranged in an amorphous SiO<NUM> matrix <NUM>. The ZrO<NUM> crystals form the cores <NUM>. The rims <NUM> comprises an intergranular phase (IGF) that comprises SiO<NUM>, ZrO<NUM>, and at least one hardness enhancing additive oxide. Most of the cores <NUM> are arranged close to another core <NUM> forming a grain boundary (GB) <NUM> in at least one direction. The GBs comprise IGF. The microstructure is further shown in for example the scanning transmission electron microscopy (STEM) micrographs in <FIG> and b. In <FIG> the parts with darker contrast are the ZrO<NUM> crystals (the cores <NUM>), while the 'background' with brighter contrast corresponded to the amorphous SiO<NUM> matrix <NUM>. In <FIG>, the parts with bright contrast are ZrO<NUM> crystals (the cores <NUM>) while the darker background is amorphous SiO<NUM> matrix <NUM>.

As a general description a nanocrystalline glass ceramic material with core-rim structure is provided that comprises an amorphous SiO<NUM> matrix <NUM>, ZrO<NUM> crystals and hardness-enhancing additive wherein the ZrO<NUM> crystals are present in cores <NUM> that are at least partly surrounded by a rim <NUM>, wherein the rim <NUM> comprises the hardness-enhancing additive.

As described above, the at least one hardness enhancing additive is present in the rims <NUM>, i.e. in the IGF in the GBs between at least two adjacent crystals (i.e. cores <NUM>) and on the surface of the crystals (i.e. cores <NUM>). In other words the at least one hardness enhancing additive , for example in the form of an oxide, exists at the ZrO<NUM>/ZrO<NUM> interface and at the ZrO<NUM>/SiO<NUM> interface. The concentration in weight% of at least one hardness enhancing additive is higher in the GB than in the ZrO<NUM>/SiO<NUM> interface.

At least in terms of hardness it is an advantage that the hardness-enhancing additive oxide in the GBs is amorphous, and/or that the IGF is amorphous. <FIG> shows STEM micrographs of connecting ZrO<NUM> crystals (brighter parts) arranged in a SiO<NUM> matrix (darker part). In between the connecting ZrO<NUM> crystals is an IGF.

In a glass ceramic material according to the invention at least a majority of the ZrO<NUM> crystals are single crystalline, meaning that the crystals themselves does not comprise any GBs. This may be advantegous in terms of optical properties.

A glass ceramic material according to the invention comprises a majority of ZrO<NUM>, as compared to the other components, in terms of both molar% and wt%. In one embodiment a nGC material comprise <NUM>-<NUM> molar% of ZrO<NUM> or <NUM>-<NUM> weight% of ZrO<NUM>. The ZrO<NUM> crystals, i.e. the cores <NUM>, form a network in the amorphous SiO<NUM> matrix <NUM>, wherein the majority of the ZrO<NUM> crystals are connected to at least one other ZrO<NUM> crystal forming GBs <NUM> in at least one direction. Without being bound to any theory, the network of ZrO<NUM> crystals connected by GBs comprising IGF may act as a structural reinforcer to the material so that the hardness of the material increases.

A glass ceramic material according to the invention is schematically illustrated in <FIG> and b. In one embodiment a glass ceramic material illustrated in <FIG> comprises an amorphous SiO<NUM> matrix <NUM>, ZrO<NUM> crystals and hardness-enhancing additive. The cores <NUM> are comprised of one or more ZrO<NUM> crystals that are at least partly surrounded by a rim <NUM>, which comprises hardness-enhancing additive.

The hardness-enhancing additive is a chemical element or composition or a mixture of different compositions or elements, e.g. Al<NUM>O<NUM> or Y<NUM>O<NUM>. In particular it may be an oxide or a mixture of oxides. The hardness-enchancing additive may be present as nano-sized domains adjacent to the ZrO<NUM> crystals, i. e the cores <NUM>. , i.e. not as a continuous ring but rather as connecting areas of hardness-enhancing additive oxide, or IGF, surrounding the ZrO<NUM> crystal, or core <NUM>. This can further be seen in the STEM micrograph in <FIG> for example wherein nanodomains of Al<NUM>O<NUM> are arranged adjacent to the ZrO<NUM> crystal.

Without being bound by any theory, the IGF arranged at the rim <NUM>, i.e. at the surface of the ZrO<NUM> crystals and in the GBs, may act as an interface enhancement such that the ZrO<NUM> stays inside the crystals and does not migrate to the amorphous matrix <NUM>.

ZrO<NUM> can exist in three different phases: monoclinic (m), tetragonal (t) and cubic (c). The m-ZrO<NUM> phase is the most stable of these. It is formed at room temperature and transitions to the c-ZrO<NUM> phase at higher temperatures. The transition occur via the t-ZrO<NUM> phase. While m-ZrO<NUM> is the most stable phase, t-ZrO<NUM> is mechanically the strongest phase and hence suitable to have in a dental material. A nGC material according to the invention may comprise ZrO<NUM> in the form of t-ZrO<NUM> and m-ZrO<NUM>, the majority being t-ZrO<NUM>. In one embodiment a nGC material may comprise <NUM>-<NUM> wt% t-ZrO<NUM> and <NUM>-<NUM> wt% of m-ZrO<NUM>. In one embodiment the zirconium dioxide comprises either tetragonal zirconium dioxide or a mixture of tetragonal zirconium dioxide and monoclinic zirconium dioxide, wherein at least <NUM> % of the zirconium dioxide crystals are tetragonal zirconium dioxide, as determined by Rietveld refinement.

Without being bound by any theory, the t-ZrO<NUM> crystals may be stabilized in their tetragonal phase by the amorphous SiO<NUM> matrix. It is advantegous in dental applications that a glass ceramic material comprises a majority of t-ZrO<NUM> as compared to the other ZrO<NUM> phases. In further embodiments, the ZrO<NUM> comprises t-ZrO<NUM>, for example <NUM>-<NUM> wt% and <NUM>-<NUM> wt% of m-ZrO<NUM>. The amount of the different crystalline phases can for example be determined by Rietveld refinement of powder X-ray diffractogram data.

In one embodiment of the first aspect of the invention, the glass ceramic material comprises <NUM>-<NUM> molar% of hardness-enhancing additive oxide. In other embodiments, the glass ceramic material comprises <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive, or <NUM>-<NUM> molar% additive.

In one embodiment of the first aspect of the invention, the glass ceramic material comprises <NUM>-<NUM> weight% of hardness-enhancing additive, preferably in the form of an oxide. In other embodiments, the glass ceramic material comprises <NUM>-<NUM> weight% additive, or <NUM>-<NUM> weight% additive, or <NUM>-<NUM> weight% additive, or <NUM>-<NUM> weight% additive, or <NUM>-<NUM> weight% additive, preferably in the form of yttrium oxide or aluminum oxide, or a combination thereof.

The hardness-enhancing additive(s) in the final material relates to the molar% or weight% of hardness-enhancing additive in oxide form. In some embodiments it relates to Y<NUM>O<NUM> or Al<NUM>O<NUM> in the material, or a combination thereof. In one embodiment the hardness-enhancing additive comprises aluminum oxide or yttrium oxide, or a combination of aluminum oxide and yttrium oxide.

In one embodiment of the first aspect of the invention the glass ceramic material comprises ZrO<NUM> crystals, i.e. cores <NUM>, wherein at least <NUM> %, or at least <NUM> % of the crystals have an average crystal size of <NUM> or less. The ZrO<NUM> crystals may be approximately <NUM>-<NUM> or <NUM>-<NUM> in diameter at the longest diameter. The diameter of the ZrO<NUM> crystals, i.e. the size of the crystals, may impact both the optical properties and the mechanical properties. For example, crystals larger than <NUM> may result in a more opaque nGC material. For at least that reason it may be advantegous that the ZrO<NUM> crystals are not too large, i.e. that the diameters of the ZrO<NUM> crystals are <NUM> or less. The crystal size, i.e. the diameter, can be determined using for example pXRD or TEM or anyother suitable technique.

In one embodiment the cores <NUM> are at least partly surrounded by rims <NUM>, such that at least <NUM> vol% of the circumference of a core <NUM> is in contact with a rim <NUM>. The rim <NUM> is approximately <NUM>-<NUM> thick and comprises at least one type of hardness-enhancing additive, oxygen, zirconium, and silicon. The concentration in wt% of hardness-enhancing additive is higher in the rim <NUM> than in the other parts of the composition. The hardness-enhancing additive is a compound or compounds, preferably in oxide form, other than SiO<NUM> and ZrO<NUM>. It can also be a combination of compounds.

The ZrO<NUM> crystals, i.e. the cores <NUM>, may comprise up to <NUM> mol%, of the hardness-enhancing additive due the solid solution between Zr and the hardness-enhancing additive.

In one embodiment of the first aspect, the ZrO<NUM> crystals, i.e. the cores <NUM>, have an ellipsoidal shape. In one embodiment of the first aspect, the majority of the ZrO<NUM> crystals, i.e. the cores <NUM>, have an ellipsoidal form, i.e. an ellipsoidal morphology. In another embodiment of the first aspect, the ZrO<NUM> crystals comprises t-ZrO<NUM>.

An ellipsoidal morphology may enable an improved package density as compared with e.g. a spherical morphology. It may also enable an improved 3D network of ZrO<NUM> in the SiO<NUM> matrix <NUM>, which may increase the hardness of the nGC material.

In one embodiment of the first aspect, the hardness-enhancing additive comprises aluminum oxide or yttrium oxide, or a combination of aluminum oxide and yttrium oxide.

In one embodiment of the first aspect, the intragranular phase is amorphous.

In one embodiment of the first aspect, the IGF comprises yttrium (Y) cations, or aluminium (Al) cations, or their respective oxides (Y<NUM>O<NUM> and Al<NUM>O<NUM>), or a combination of either of these. It is advantegous at least in terms of hardness of the glass ceramic material that the IGF is amorphous. Both Y<NUM>O<NUM> and Al<NUM>O<NUM> are advantegous to use as a hardness-enhancing additive since they may increase the hardness and the Young's modulus of a nGC material.

In one embodiment of the first aspect, the IGF comprises manganese (Mn) cations, or magnesium (Mg) cations, or cerium (Ce) cations or any of their respective oxides (MnO, Mn<NUM>O<NUM>, Mn<NUM>O<NUM>, MnO<NUM>, MnO<NUM>, Mn<NUM>O<NUM>, MgO, Ce<NUM>O<NUM>, Ce<NUM>O<NUM> and CeO<NUM>), or a combination of either of these. It is advantegous at least in terms of hardness of the glass ceramic material that the IGF is amorphous. The amounts and preferred amounts of these hardness-enhancing additives (Mn, Mg, and Ce) are the same as given for Y<NUM>O<NUM> or Al<NUM>O<NUM>, when taking into consideration the number of cations per mol for each of the additives of the finished product. For example, if the amount of Y<NUM>O<NUM> in the finished product would be <NUM> molar%, or <NUM>-<NUM> wt% then the corresponding amount of MnO would be <NUM> molar%, or <NUM>-<NUM> wt% and for CeO<NUM> it would be <NUM> molar%, or <NUM>-<NUM> wt%.

In one example of the first aspect, the IGF comprises yttrium (Y) cations, or aluminium (Al) cations, or its respective oxides (Y<NUM>O<NUM> and Al<NUM>O<NUM>), or a combination of either of these, in combination with one or more of manganese (Mn) cations, magnesium (Mg) cations, cerium (Ce) cations or any of their respective oxides (MnO, Mn<NUM>O<NUM>, Mn<NUM>O<NUM>, MnO<NUM>, MnO<NUM>, Mn<NUM>O<NUM>, MgO, Ce<NUM>O<NUM>, Ce<NUM>O<NUM> and CeO<NUM>).

In one embodiment of the first aspect, the IGF comprises yttrium oxide.

In one embodiment of the first aspect, the nGC comprises amorphous Y<NUM>O<NUM> as hardness-enhancing additive, forming a Y<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material.

In a Y<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material, the ZrO<NUM> may be in the form of crystalline particles, <NUM>, embedded in an amorphous SiO<NUM> matrix <NUM>. The yttrium component (e.g. cation or oxide) in the Y<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material is present in the rim <NUM>. It may also be present inside the ZrO<NUM> crystal lattice, as discussed above.

In one embodiment of the first aspect, the nGC material comprises <NUM>-<NUM> mol% Y<NUM>O<NUM>, <NUM>-<NUM> mol% SiO<NUM> and <NUM>-<NUM> mol% ZrO<NUM>, or <NUM>-<NUM> wt% Y<NUM>O<NUM>, <NUM>-<NUM> wt% SiO<NUM>, and <NUM>-<NUM> wt% ZrO<NUM>.

In one embodiment of the first aspect, the hardness-enhancing additive comprises aluminum oxide.

In one embodiment of the first aspect the nGC comprises Al<NUM>O<NUM> as hardness-enhancing additive, forming a Al<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material.

In an Al<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material the ZrO<NUM> may be in the form of crystals, i.e. cores <NUM>, embedded in an amorphous SiO<NUM> matrix <NUM>. The Al<NUM>O<NUM> in the Al<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> material is present in the rim <NUM>. It may additionally be present to some extent in the SiO<NUM> matrix <NUM>.

In one embodiment of the first aspect the nGC material comprises <NUM>-<NUM> mol% Al<NUM>O<NUM> or <NUM>-<NUM> mol% Al<NUM>O<NUM>, <NUM>-<NUM> mol% SiO<NUM> and <NUM>-<NUM> mol% ZrO<NUM>, or <NUM>-<NUM> wt% Al<NUM>O<NUM>, <NUM>-<NUM> wt% SiO<NUM>, and <NUM>-<NUM> wt% ZrO<NUM>.

One advantage of nGC materials comprising Al<NUM>O<NUM> as a hardness-enhancing additive in the IGF is that the Al<NUM>O<NUM> may increase the fracture toughness of the nGC material. Fracture toughness is a way of expressing the material's resistance to crack propagation.

All variants and examples of the first aspect can be combined with the second and third aspects unless expressly stated otherwise.

In a second aspect of the invention there is a method of forming a glass ceramic wherein the method comprises the following steps:.

The fractionation step of the method relates to reducing the particle size of the material. This may achieved by using the drying method in step <NUM> that reduces the particle size or by milling the xerogels formed after step <NUM> or <NUM>. In such a case the xerogel will be formed into a powder.

A nGC material according to the invention may be prepared in a sol-gel process. In such a sol-gel process, a powder is formed, which may subsequently be sintered to form a final structure.

Sol-gel processes or methods are well-known techniques in inorganic chemistry to form solid materials from small molecules. The process generally involves the conversion of monomers into a colloidal solution, i.e. a sol, that acts as the precursor for an integrated network, i.e. a gel. The gel may be composed of small particles or a network of polymers.

After a sol-gel process, an inorganic, solid, powder is formed. The formed powder may be sintered to form a solid body. Sintering is a process, or method, of compacting and forming a solid from a powder by heat and/or pressure without including melting. The powder formed in the sol-gel process may be compacted with or without heating prior to being sintered. It may also be compacted during sintering.

A nGC material according to the invention can be synthesized using a sol-gel method comprising the following sequential steps, see <FIG>:.

In one embodiment, step <NUM>' or <NUM>' is part of the process. In other embodiment, if e.g. a different drying process is used in step <NUM>, then step <NUM>' or <NUM>' may become optional if said reduction of particle size (fractionation) is achieved in the drying process. Examples of different drying processes are evaporation of the sol or spray drying.

Examples of SiO<NUM> precursor materials in step <NUM> are tetraethyl orthosilicate (TEOS), ethyl silicate, and silicon alkoxides. Examples of ZrO<NUM> precursor materials in step <NUM> are zirconium propoxide or Zr(OPr)<NUM>, zirconyl nitrate, ZrOCl<NUM> solution, and zirconium (IV) chloride.

The catalyst may be acidic. Examples of suitable catalysts are hydrochloride acid, nitric acid, citric acid, acetic acid, ethylenediaminetetraacetic acid, tartaric acid, glycolic acid, oxalic acid, malic acid, and formic acid.

Examples of precursor materials for additives are Al(O-i-Pr)<NUM> for aluminium and YCl<NUM> for yttrium. Other examples include aluminium-sec butoxide or Al(OBu)<NUM>, AlCl<NUM>, Al(NO<NUM>)<NUM>·<NUM><NUM>O and Y(NO<NUM>)<NUM>, yttrium acetate, yttrium oxo-isopropoxide or Y<NUM>O(OPri)<NUM>, Y<NUM>(SO<NUM>)<NUM>, yttrium isopropoxide or C<NUM>H<NUM>O<NUM>Y.

The precursor material for the hardness-enhancing additive may constitute <NUM>-<NUM> molar% of the total amount of the precursor materials ZrO<NUM>, SiO<NUM>, and hardness-enhancing additive, calculated as the content of Y<NUM>+ or Al<NUM>+ or a combination thereof. In one embodiment of the second aspect of the invention, the glass ceramic material comprises <NUM>-<NUM> molar% hardness-enhancing additive. In other embodiment, the glass ceramic material comprises <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive, or <NUM>-<NUM> molar% hardness-enhancing additive.

The amounts and preferred amounts of the other precursors for the hardness-enhancing additives (Mn, Mg, and Ce) are the same as given for Y<NUM>O<NUM> or Al<NUM>O<NUM>, when taking into consideration the number of cations per mol for each of the precursor materials for the hardness-enhancing additives. For example, if the amount of Y<NUM>+ in the precursor material for the hardness-enhancing additive would be <NUM> molar%, then the corresponding amount of Mn<NUM>+ would be <NUM> molar%.

Different solvents can be used in the sol-gel method <NUM>, for example ethanol, methanol, anhydrous <NUM>-propanol and isopropanol. It is important to control the pH in the different steps of the sol-gel method, in order to at least partly control the gelation process and to dissolve the precipitates. This can for example be done by dripwise addition of HCl to the sol(s) and also during the different steps of the reaction. The rate of the dripping may be varied.

In one embodiment of the second aspect, step <NUM> may comprise additional steps <NUM>', <NUM>", and <NUM>"', see <FIG> uses the same marks as <FIG>, i.e. the two sols are marked A and B, the additive is marked C, gel is marked X, xerogel is marked Y, and solid product is marked Z.

Step <NUM>' comprises heating, wherein a pseudogel (marked R in <FIG>) is formed after the two sols have been mixed and heated. It may automatically form a clear sol after some time, but in in optional step <NUM>", the pseudogel is kept at an elevated temperature, e.g. <NUM>-<NUM>, for a time period of <NUM>-<NUM> after which it forms a clear sol (marked Q in <FIG>). In optional step <NUM>"', an acid is added to the clear sol in step <NUM>"', after which a gel is formed that is dried in step <NUM> to a xerogel.

In one embodiment of the second aspect, the sintering in step <NUM> of the method is performed using hot isostatic pressure (HIP), or spark plasma sintering (SPS).

SPS has a high heating rate as compared to other sintering techniques, which can be used to limit or avoid grain growth at relatively low sintering temperatures.

HIP and HP are other manufacturing techniques that can be used to increase the density of a ceramic material. This is achieved by applying both heat and pressure.

Before the sintering step, the powder may be compacted to increase the density of the material after sintering. Optionally, the packing density can be further enhanced by addition of a step of granulation before the compaction and/or sintering. Examples of methods of powder granulation are spray drying, extrusion and spherinizer, reverse wet granulation, steam granulation, moisture-activated dry granulation, thermal adhesion granulation, freeze granulation, foam granulation. The granulation method should form granules having a granule size of <NUM> to <NUM>.

All variants and examples of the second aspect can be combined with the first and third aspect unless expressly stated otherwise.

In a third aspect of the invention, there is a densified material comprising a glass ceramic material.

In general, in the dental art, a densified material has the meaning of a material that has a density of <NUM> % or more of the theoretical density.

In the third aspect of the invention there is a densified glass ceramic material comprising ZrO<NUM> crystals, forming cores <NUM>, arranged in an amorphous SiO<NUM> matrix <NUM>, wherein the ZrO<NUM> crystals are surrounded by a rim <NUM> comprising an IGF that comprises at least one hardness-enhancing additive. Such a material may be prepared in a method comprising steps <NUM>-<NUM>.

Densification is a process of reducing the porosity in a material, i.e. making it denser. The sintering process controls the densification. Densification of a material usually occurs at high temperatures, for example <NUM>-<NUM>, or <NUM>-<NUM>. In one embodiment of the third aspect, there is a dental restorative material comprising a densified material. The densified material is shaped into the shape of a human tooth and used as a dental restorative material. Such dental restorative material comprises a glass ceramic material according to the invention, comprising an amorphous SiO<NUM> matrix <NUM>, ZrO<NUM> crystals in the form of cores <NUM> and a rim <NUM> comprising an IGF that comprises at least one hardness-enhancing additive. The ZrO<NUM> crystals in the cores <NUM> are at least partly surrounded by the rim <NUM>.

All variants and examples of the third aspect can be combined with the first and second aspect unless expressly stated otherwise.

Three different sets of glass ceramic materials were prepared, one comprising yttrium as hardness-enhancing additive and one comprising aluminium as hardness-enhancing additive and one with no additive. All materials were prepared using the method comprings steps <NUM>-<NUM> illustrated in <FIG>. All formed materials were evaluated in terms of mechanical and optical properties.

A fourth set of material were prepared comprising a mixture of hardness-enhancing additive. The fourth set were evaluated in terms of mechanical properties.

Synthesis:. Four different Y<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> samples (named Y-<NUM> to Y-<NUM>) were fabricated by a sol-gel method wherein tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, St Louis, MO, USA) and zirconium n-propoxide Zr(OPr)<NUM> (<NUM> wt% in <NUM>-propanol from Sigma-Aldrich) were used as the starting alkoxide precursors materials for SiO<NUM> and ZrO<NUM>, respectively. HCl was added as a catalyst. YCl<NUM> powder was added into the mixed sol before final hydrolysis and polymerization. YCl<NUM> dissolved in the sol, did not show significant effect on the hydrolysis and polymerization processes of the mixed sol. The obtained sol-gel powder was calcined at <NUM> for <NUM> in a muffle furnace to remove organics from precursors. Disc samples were obtained by hot pressing (Y1) or SPS (Y2-Y4), with a holding temperature of <NUM>, a holding time of <NUM>, and an applied pressure of <NUM> MPa. The samples had the following compositions, in molar% and in weight%:.

The contents in molar% of the precursor materials were:.

Sample Y-<NUM> was analyzed in terms of phase analysis and microstructure. Samples Y-<NUM>, Y-<NUM> and Y-<NUM> were analyzed in therms of mechanical properties.

Material characterization: The phase analysis was performed by X-ray diffraction (XRD) on a D8 Advanced diffractometer (Bruker Corporation, Billerica, MA). The data were acquired with Ni-filtered Cu Kα radiation (<NUM> kV, <NUM> mA) in the 2θ interval between <NUM> and <NUM>°, with a scan step of <NUM>/step and a size of <NUM>°. The quantitative phase composition analysis was obtained from Rietveld refinement using Profex software. For transmission electron microscopy (TEM) analysis, an electron transparent lamella of the Y-<NUM> and Y-<NUM> samples was prepared with a dual beam focused ion beam-scanning electron microscope (FIB-SEM, FEI Strata DB325) and attached to Cu lift-out grid. The analysis was carried out on a probe corrected FEI Titan Themis equipped with the SuperX system for energy dispersive X-ray spectroscopy (EDS). The EDS elemental maps were acquired and quantified with the Esprit software developed by Bruker.

The mechanical properties of the samples were evaluated in terms of Young's modulus, nanohardness, microhardness and fracture toughness. The Young's modulus and hardness measurements were carried out on a nanoindentation tester (Ultra nanoindenter, CSM instruments) with a load of <NUM>µN at a speed of <NUM>µN/min. <NUM> indentations with proper distance from each other were performed for each sample. Young's modulus was calculated according to Oliver-Pharr method (equation <NUM>): <MAT> where y is the Poisson ratio, β is the Oliver-Parr constant, Su is the slope at start of the unloading curve, A is indenter area function, γi and Ei are Poisson's ratio and Young's modulus of indenter material, respectively.

A microhardness tester (Buehler Micromet <NUM>, Lake Bluff, IL, USA) was used to measure the Vickers hardness on the micro scale with an indentation load of <NUM> N. <NUM> indentations were preformed on each sample. The length of crack and indentation diagonal were measured using the equipped software on the instrument. The fracture toughness was calculated by the Palmquvist method.

Results: XRD pattern, <FIG>, for the Y-<NUM> sample shows that the Y-<NUM> sample was mainly composed of t-ZrO<NUM>, while small diffraction peaks belonging to m-ZrO<NUM> phase were also found (at <NUM>° and <NUM>°), indicating that a certain amount of t-ZrO<NUM> transformed to m-ZrO<NUM> during the sintering process. SiO<NUM> was X-ray amorphous since no obvious peaks belonging to crystalline forms were found in the XRD patterns. Quantification results by Rietveld refinement demonstrated that the Y-<NUM> sample contained <NUM>. 6wt% t-ZrO<NUM> and <NUM>. 4wt% m-ZrO<NUM>.

The microstructure of the Y-<NUM> sample was characterized by scanning transmission electron microscopy (STEM) technique. STEM-bright field (BF) images (<FIG>) show an overview of the microstructure. The parts with darker contrast are ZrO<NUM> crystals, while the 'background' with brighter contrast corresponded to the amorphous SiO<NUM> matrix. Thus, the basic structural characteristic of the Y-<NUM> sample is ZrO<NUM> crystals embedded in an amorphous SiO<NUM> matrix. The majority of the ZrO<NUM> crystals had an ellipsoidal morphology, i.e. shape, and their sizes generally ranged from <NUM> to <NUM> in diameter. In <FIG>, particles with bright contrast are ZrO<NUM> crystals, since in the STEM-high angle annular dark field (HAADF) imaging mode the contrast is proportional to the atomic number (Z), and heavier atoms appear brighter. Most of the ZrO<NUM> crystals, were connected with their adjacent crystals by grain boundary in at least one direction. The crystals and matrix were confirmed as ZrO<NUM> and SiO<NUM>, respectively, by STEM-EDS maps (not shown). Oxygen elements was nearly homogenously distributed in both ZrO<NUM> crystals and SiO<NUM> matrix, with slightly concentrated distribution in ZrO<NUM> crystals. Y element distributed both around and within the ZrO<NUM> crystals. Slightly more intense Y signals were detected at the ZrO<NUM>/SiO<NUM> interfaces and at the grain boundaries between ZrO<NUM> crystals, indicating that there was concentrated Y distribution at those two regions. No obvious Y signal were detected in the SiO<NUM> matrix.

The elemental distribution of the grain boundary appearence with a layer of intergranular phase was examined with STEM-EDS line scanning and the results are shown in <FIG>and <FIG>. It can be observed that Y segregated in the intergranular phase (IGF, marked I in <FIG> and <FIG>), i.e. in the area in between the adjecent ZrO<NUM> crystals. In <FIG> the different elements have different marks: ■ is for O, ☆ is for Si, ◆ is for Zr, and ▼ is Y. In <FIG> ■ is for Si and ● is for Y. <FIG> is a STEM-HAADF image showing connected ZrO<NUM> crystals and a thin layer of IGF between the ZrO<NUM> crystals. A STEM-EDS line scan was carried out along the white arrow line and the distribution of O, Si, Y, and Zr elements along the white arrow line is shown in <FIG>, from which it can be observed that Y and Si elements showed higher atomic ratio between point <NUM> and point <NUM>, indicating that this region was rich in Y and Si. Meanwhile, the atomic ratio of Zr in this region was lower than in other regions. O elements showed homogenous distribution along the scan line, without obvious rich or deficient regions. To further analyze the distribution of Y and Si along the scan line, the data of these two elements were plotted separately as displayed in <FIG> shows that the IGF was rich in Y (marked I in <FIG>).

Samples Y-<NUM>, Y-<NUM> and Y-<NUM> were evaluated in terms of mechanical properties. The results are summarized in Table <NUM>, together with the wt% for the compositions.

Synthesis: Three different Al<NUM>O<NUM>-ZrO<NUM>-SiO<NUM> samples were prepared using a sol-gel method wherein tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, St Louis, MO, USA) and zirconium n-propoxide Zr(OPr)<NUM> (<NUM> wt% in <NUM>-propanol from Sigma-Aldrich) were used as the starting alkoxide precursor materials for SiO<NUM> and ZrO<NUM>, respectively. HCl was added as a catalyst. Al(O-i-Pr)<NUM> powder was added into the mixed sol before final hydrolysis and polymerization. Al(O-i-Pr)<NUM> dissolved in the sol, and did not show significant effect on the hydrolysis and polymerization processes of the mixed sol. The obtained sol-gel powder was calcined at <NUM> for <NUM> in a muffle furnace to remove organics from precursors. Disc samples were obtained by SPS, with a holding temperature of <NUM>, a holding time of <NUM>, and an applied pressure of <NUM> MPa. The samples had the following compositions, in molar% and in wt%:.

Material characterization: The phase analysis of the samples was performed by X-ray diffraction (XRD) on a D8 Advanced diffractometer (Bruker Corporation, Billerica, MA). The data were acquired with Ni-filtered Cu Kα radiation (<NUM> kV, <NUM> mA) in the 2θ interval between <NUM> and <NUM>°, with a scan step of <NUM>/step and a size of <NUM>°. For transmission electron microscopy (TEM) analysis, an electron transparent lamella of the Al-<NUM> sample was prepared with a dual beam focused ion beam-scanning electron microscope (FIB-SEM, FEI Strata DB325) and attached to Cu lift out grid. The analysis was carried out on a probe corrected FEI Titan Themis equipped with the SuperX system for energy dispersive X-ray spectroscopy (EDS). The EDS elemental maps were acquired and quantified with the Esprit software developed by Bruker.

A microhardness tester (Buehler Micromet <NUM>, Lake Bluff, IL, USA) was used to measure the Vickers hardness on the micro scale with an indentation load of <NUM>. <NUM> indentations were preformed on each sample. The length of crack and indentation diagonal were measured using the equipped software on the instrument. The fracture toughness was calculated by the Palmquvist method.

Results: XRD patterns, <FIG>, for the bulk samples after hot pressing show that all samples were mainly composed of t-ZrO<NUM>, <FIG> shows sample Al-<NUM>, <FIG> shows sample Al-<NUM>, and <FIG> show sample Al-<NUM>, while small diffraction peaks belonging to m-ZrO<NUM> phase were also found (at <NUM>° and <NUM>°) in samples Al-<NUM>, see <FIG>, and Al-<NUM>, see <FIG>, indicating that a certain amount of t-ZrO<NUM> transformed to m-ZrO<NUM> during the sintering process. In all three samples, SiO<NUM> was X-ray amorphous since no obvious peaks belonging to crystalline forms were found in the XRD patterns.

The microstructure of the Al-<NUM> sample was characterized by scanning transmission electron microscopy (STEM) technique, <FIG>. STEM-bright field (BF) image (<FIG>) demonstrated an overview of the microstructure. The parts with darker contrast were ZrO<NUM> crystals, while the 'background' with brighter contrast corresponded to amorphous SiO<NUM> matrix, see <FIG>. Thus, the basic structural characteristic of the Al-<NUM> sample was ZrO<NUM> crystals embedded in an amorphous SiO<NUM> matrix. The majority of ZrO<NUM> crystals had an ellipsoidal morphology and their diameters generally ranged from <NUM> to <NUM>. In <FIG>, particles with bright contrast were ZrO<NUM> crystals, since in the STEM-high angle annular dark field (HAADF) imaging mode, the contrast is proportional to the atomic number (Z), and heavier atoms appear brighter. Most of the ZrO<NUM> crystals were connected with their adjacent particles by grain boundary in at least one direction. The crystals and matrix were confirmed as ZrO<NUM> and SiO<NUM>, respectively, by STEM-EDS maps (not shown). Oxygen elements was nearly homogenously distributed in both the ZrO<NUM> crystals and the SiO<NUM> matrix, with slightly concentrated distribution in the ZrO<NUM> crystals. The Al element was distributed around the ZrO<NUM> crystals. More intense Al signals were detected at the ZrO<NUM>/SiO<NUM> interfaces and at the grain boundaries between adjecent ZrO<NUM> crystals, indicating that there was concentrated Al distribution at those two regions. No obvious Al signal was detected in the SiO<NUM> matrix or in the ZrO<NUM> crystals.

The elemental distribution of the grain boundary with a layer of intergranular phase was examined with STEM-EDS line scanning and the results are shown in <FIG>. It can be observed in <FIG>that Al segregated in the intergranular phase (IGF). <FIG> is a STEM-HAADF image showing connected ZrO<NUM> crystals and a thin layer of IGF between the ZrO<NUM> crystals. STEM-EDS line scans were carried out along the the black line arrows marked "Line scan <NUM>", "Line scan <NUM>" and "Line scan <NUM>". <FIG> shows the results from Line scan <NUM> showing that region between approximately <NUM> and <NUM> in distance had a high concentration of Al (line · · , i.e. dotted) and low concentration of Si (line - · -, i.e. dashed/dotted) and Zr (line - - -, i.e. dashed). <FIG> shows the results from Line scan <NUM> showing that region between <NUM> and <NUM> in distance had a high concentration of Al than the region below <NUM> and high <NUM>, the lines in <FIG> has the same marks as in <FIG>. <FIG> shows the results from Line scan <NUM>, i.e. the IGF region, showing that the region between <NUM> and <NUM> in distance had a high concentration of Al. The lines in <FIG> has the same marks as the lines in <FIG>and <FIG> with the addition that the solid line represent O.

All three samples were evaluated in terms of mechanical properties, the results are summarized in Table <NUM>, together with the wt% of the compositions.

Synthesis: Three different ZrO<NUM>-SiO<NUM> samples without hardness-enhancing additive were prepared for comparative testing. The samples were prepared using a sol-gel method wherein tetraethyl orthosilicate (TEOS) (Sigma-Aldrich, St Louis, MO, USA) and zirconium n-propoxide Zr(OPr)<NUM> (<NUM> wt% in <NUM>-propanol from Sigma-Aldrich) were used as the starting alkoxide precursor materials for SiO<NUM> and ZrO<NUM>, respectively. The obtained sol-gel powder was calcined at <NUM> for <NUM> in a muffle furnace to remove organics from precursors. Disc samples were obtained by SPS, with a holding temperature of <NUM>, a holding time of <NUM>, and an applied pressure of <NUM> MPa. The samples had the following compositions, in molar%:.

The three SiZr samples were analysed for phase composition and mechanical properties as describes in Examples <NUM> and <NUM>. The phase analysis showed that the materials were mainly composed of t-ZrO<NUM>. The mechanical properties are shown in Table <NUM> below together with the wt% of the compositions.

A set of additional ZrO<NUM>-SiO<NUM> samples were prepared comprising MnOx and Al<NUM>O<NUM> as hardness-enhancing additive. Four samples were prepared with MnOx content ranging from <NUM> to <NUM> wt% (<NUM> to <NUM> molar%). The samples were sintered using pressure less with a holding temperature of <NUM>, a ramping rate of <NUM>/min, and a holding time of <NUM> -<NUM> hours with <NUM> samples/group.

The biaxial strength of the samples were tested. The results can be seen in Table <NUM> below.

Claim 1:
A glass ceramic material comprising zirconium dioxide crystals embedded in an amorphous silicon dioxide matrix (<NUM>) and at least one hardness enhancing additive comprising aluminum oxide or yttrium oxide, or a combination of aluminum oxide and yttrium oxide in a concentration of <NUM>-<NUM> weight%, wherein the zirconium dioxide crystals form cores (<NUM>) and the cores (<NUM>) are at least partly surrounded by rims (<NUM>), wherein the rims (<NUM>) comprise an intergranular phase, and wherein the intergranular phase comprises at least silicon dioxide, zirconium dioxide and at least one hardness-enhancing additive, and wherein the concentration in weight percent of the hardness-enhancing additive is higher in the rims (<NUM>) than in the amorphous silica matrix (<NUM>) and in the cores (<NUM>).