T 0444/91 (Zirconia porcelain/NGK INSULATORS) 18-07-1997

Reasons for the Decision

1. The appeal is admissible.

2. The Board notes that during the appeal proceedings all the documents quoted under point II above were discussed by each of the parties. Accordingly, the Board considers each of them to have been introduced into the proceedings.

3. Claim 1, relating to a zirconia porcelain, is a combination of claims 1, 2, 3, 5 and 6 as originally filed, corresponding to claims 1, 2, 3, 5 and 6 as granted. Dependent claim 2 corresponds to claim 4 both as granted and as originally filed. Claim 3, relating to a method of producing a zirconia porcelain, is based on claim 7 both as granted and as originally filed and contains each of the features of the zirconia porcelain according to present claim 1. Dependent Claims 4 and 5 correspond to claims 9 and 10 both as granted and as originally filed. The scope of the new claims is narrower than that of the claims as granted, and the Board considers that the requirements of Articles 123(2) and (3) EPC are satisfied.

4. In the grounds of opposition the Appellant made reference to the content of the monoclinic phase of the zirconia porcelain according to claims 5 and 6 as granted and argued that document (1) would disclose a diagram of the zirconium oxide, yttrium oxide and cerium oxide system which diagram did not allow the presence of a monoclinic phase having regard to the stabiliser composition claimed in the patent in suit and therefore, in the light of the teaching of said claims 5 and 6, the skilled person could not carry out the invention.

This objection under Article 83 EPC was not further substantiated.

It is established Board of Appeal case law that the question of sufficient disclosure is not to be judged merely on the basis of the claims (see e.g. T 14/83, OJ EPO 1984, 105). There cannot be any doubt that the examples of the patent in suit show how a zirconia porcelain as defined in claim 1 (now comprising the features of claims 5 and 6 as granted) can be obtained. Thus, the requirements of Article 83 EPC are also met.

5. The patent in suit relates to a porcelain material containing tetragonal, cubic and monoclinic zirconium oxide having, according to the description, a high strength, excellent thermal stability, suffering little or not at all from deterioration phenomena such as shape change, reduction in strength and the like even in long-term use and exhibiting excellent durability (see page 2, lines 23 to 25). The Board agrees with the Opposition Division's view that for such a zirconia porcelain in a so-called "non-equilibrium state" document (6) represents the closest prior art. This was not disputed by the parties at the appeal stage.

5.1. Document (6) discloses a zirconia ceramic comprising ZrO2 and Y2O3 in a molar ratio of Y2O3/ZrO2 of 2/98-7/93 and consisting of crystal grains having a mixed phase comprising a tetragonal phase and a cubic phase or having a phase comprising a tetragonal phase, the average size of the crystal grains being not larger than 2 µm (see page 3, lines 16 to 24). According to page 8, lines 10 to 25, the said mixed crystal phase of the ceramic material may consist only of a tetragonal phase and a cubic phase or may comprise a tetragonal phase, a cubic phase and not more than about 20% by volume of monoclinic phase based on the total crystal phase. The zirconia ceramic may also comprise the tetragonal phase and at least one of a monoclinic phase and a cubic phase in an amount of not more than 20% by volume of the content of the said at least one of monoclinic phase and cubic phase (see page 8, line 25 up to page 9, line 3).

Yttria is mainly used as a stabiliser for zirconia. Not more than about 30 mol% of the yttria may be replaced by oxides of rare earth elements such as Yb2O3, Sc2O3, Nb2O3, and Sm2O3, or by CaO or MgO. The zirconia ceramics of document (6) may also contain not more than 30% by weight, based on the total amount of the ceramics, of sintering aids such as SiO2, Al2O3, and clay (see page 9, lines 4 to 13). In the method of producing the zirconia ceramics, the mixture of the suitable starting material is converted into a moulded article and is fired in air at a temperature within a range of 1000 C - 1550 C. It is indicated that the zirconium oxide to be molded into the article has to be zirconium oxide having a crystallite size of not larger than 1000 Angstrom in order to stably maintain the crystal phase of the resulting zirconia ceramic to be the mixed phase comprising the tetragonal phase (see pages 10, lines 21 to 34 and page 12, lines 6 to 10).

It is furthermore indicated on page 6, lines 7 to 15, with reference to curve B of Figure 1, of document (6) that after a durability test during which the ceramic is maintained for 1500 hours within a specifically limited temperature range of 200 C to 300 C , when the average crystal grain size in the final ceramic material becomes larger than 2 µm, the strength of the ceramic is noticeably decreased due to the formation of excess monoclinic phase.

Said Figure 1 shows a steep decline of said curve B for a zirconia ceramic material produced with an average crystal grain size larger than 2 µm.

As regards the formation of the monoclinic phase, it is then explained on page 6, lines 25 to 35, that the transformation from a tetragonal phase to a monoclinic phase hardly occurs in the case where the crystal grain size is small and that, when crystal grains have a very fine size, the tetragonal phase is more stable than the monoclinic phase owing to the surface free energy of the grains.

Subsequently, it is stated that the tetragonal phase greatly contributes to the good mechanical properties of zirconia ceramics.

5.2. The Board notes that the patent in suit does not comprise any worked example showing in a direct comparison an overall improvement of mechanical properties of the zirconia porcelain according to claim 1 over that known from the closest prior art according to document (6), namely when the average particle size of the zirconia crystals is below 2 µm.

Accordingly, in relation to document (6), the problem to be solved can only be seen in providing a zirconia porcelain keeping a high amount of the tetragonal phase under broader production conditions.

5.3. The solution lies in a composition with the technical features of claim 1, comprising 2 to 3 mol% of yttrium oxide stabiliser and 2 to 4 mol% of cerium oxide as rare earth element stabiliser component.

Having regard to Figure 4 of the patent in suit showing that from about 0 up to a particle size of 10 µm the flexural strength and the dimensional change of the material is not sharply dependent on the crystal grain size, the Board finds that it is credibly demonstrated that cerium oxide within the claimed range relatively decreases the influence of the particle size of zirconia crystals on the mechanical properties of the porcelain material and thus optimises the composition known from document (6) such that a porcelain material under broader production conditions may be produced by maintaining a tetragonal phase.

The Appellant did not file any counterevidence that the materials having a crystal size within the claimed range either do not contain the required tetragonal phase or show such drawbacks that the occurrence of less good mechanical properties outweighs the advantage of the possibility of having broader production conditions.

The Board notes that the Appellant did not reply to the Respondent's submission dated 19 June 1996, including explanations regarding the outcome of said Figure 4 of the patent in suit and Figure 1 according to document (6).

The provision of the possibility of broader production conditions of a zirconia porcelain attainable by the composition of the patent in suit was already discussed before the Opposition Division (see minutes of oral proceedings held on 26 September 1990, point 3).

Accordingly, the Board sees no reason to doubt that the problem has indeed been solved.

5. After examination of the cited prior art, the Board has reached the conclusion that the zirconia porcelain of claim 1 is not disclosed therein and that the claimed subject matter is therefore novel.

6. The remaining issue in this case is inventive step.

6.1. Document (6) itself does not mention the use of ceria as an oxide of a rare earth element suitable to replace certain amounts of the yttria component as a stabiliser. This document is wholly silent about any reasons for substituting certain amounts of the yttria stabilizer and comprises no other teaching than to control the fine structure of the final ceramic material so as to obtain small crystal grain sizes well below 2 µm and this prior art therefore teaches only to ensure under restricted production conditions that the critical limit for crystal size in the final product of 2. µm is not exceeded.

6.1.1. However, if confronted with the problem as stated above, the skilled person inevitably would turn to other prior art relating to zirconia porcelain material comprising a tetragonal crystal phase and first of all take into account documents particularly containing technical information with respect to a stabilisation or a retention of the tetragonal phase.

6.1.2. Document (8) describes such a ceramic material in the form of a two-phase ceramic composite, one of the phases is ZrO2 with a rare earth oxide such as Y2O3, CeO2, La2O3 and/or Er2O3 dissolved therein to maintain at least some of the phase in the tetragonal structure, the second phase being aluminium oxide. In a first preferred embodiment the rare earth oxide is Y2O3 in the range of 0.5 to 5.5 mole% of Y2O3 to Y2O3 plus ZrO2. In a second preferred embodiment the rare earth oxide is CeO2 in the range of 0.5 to 10 mole% of CeO2 to CeO2 plus ZrO2 (see column 2, lines 16 to 40). As regards the size of the zirconia crystals, document (8) contains a reference to theoretical calculations showing that a critical size exists, below which a particle of tetragonal zirconium oxide can be constrained in its transformation by an elastic matrix. It is than concluded that if yttrium oxide is held in solution in the zirconium oxide, it increases the critical grain size below which tetragonal zirconium oxide can be retained in an Al2O3/ZrO2 matrix and that consequently the tetragonal zirconium oxide can be obtained in Al2O3/ZrO2 composites with grain sizes up to about 2 µm (see column 5, lines 22 to 50). It is furthermore concluded in column 6, lines 30 to 58, with reference to phase diagrams compiled by the "National Bureau of Standards" that the addition of CeO2, La2O3 or Er2O3 to an Al2O3/ZrO2 composite would also favour the retention of tetragonal zirconium oxide in a manner analogous to the yttrium oxide addition and that mixtures of Y2O3, CeO2, La2O3 and Er2O3 could be added in various portions within the above ranges to promote the tetragonal zirconium oxide.

Even if the Board were to accept the Appellant's argument that a person skilled in the art would not distinguish between the use of an aluminium oxide component as a sintering aid in a glassy phase according to the patent in suit and the use of aluminium oxide in a crystalline phase forming a separate phase of a composite material according to document (8), the Board is unable to see on which basis the skilled person, faced with the problem defined above, in a first step would select the specific combination of Y2O3 /CeO2 and then, in a second step, would modify the parameters known from document (6) outside the critical grain size taught therein.

6.1.3. Document (4) represents a review article relating in general to oxide ceramics. The Board agrees with the Appellant that in sub-section 3.3.2 document (4) describes zirconium ceramic materials comprising a mixture of cubic, monoclinic and tetragonal zirconium oxide phases partially stabilised with rare-earth element components such as yttrium oxide and having an excellent thermal stability. However, in the absence of any data relating to production conditions of the material with respect to the grain size of the crystallites and without any hint that a specific combination of stabilisers should be used, document (4), even in combination with documents (8) and (6), could not lead a person skilled in the art to the claimed solution.

6.2. None of the other cited prior art documents is concerned with the stabilisation or retention of a tetragonal zirconium oxide material and thus provide any incentive at all to broaden the production conditions of a zirconia porcelain obligatorily comprising such a metastable phase.

6.2.1. The Appellant is right in arguing that documents (1), (2), (3) and (5) as well as document (7) make express reference to phase diagrams of the ZrO2/Y2O3 /CeO2 system and that in particular document (1) (page 1317, Figure 2) and document (5) (page 732, Figure 2) disclose ternary compositions lying slightly outside or even on the cubic phase boundary line and that document (5) expressly mentions a mixed crystal phase region of the phase diagram possibly comprising monoclinic, cubic and tetragonal zirconium oxide structures (page 733, first to third full paragraph).

However, apart from the fact that

(i) document (1) only discloses a material with a particle size around 10 µm containing 7 mol% cerium oxide and 7 mol% yttrium oxide (page 1318, Figure 4 c and page 1319, right column, last paragraph), a composition far removed from the porcelain according to claim 1, and that

(ii) document (2) only expressly describes a composition with 6 mol% yttrium oxide and 2 mol% cerium oxide and mentions neither a crystal size nor a tetragonal phase, and that

(iii) document (3) does not mention the tetragonal phase and describes a material with 12 mol% cerium oxide and 3 mol% yttrium oxide with a crystal size well below 1 µm not showing a tetragonal phase (page 1218, second paragraph, composition "I" and page 1219, seventh and last paragraph) a composition also far removed from the porcelain according to claim 1, and that

(iv) document (5) only mentions lattice parameters and is totally silent as to a particle size of the zirconium oxide crystals, and that

(v) document (7), cited by the Respondent, describes kinetics of a material with 5 mol% yttrium oxide and 7 mol% cerium oxide, also far removed from the porcelain according to claim 1,

there is not the slightest hint in any of these documents as to a selection of compositions in regions of the ternary ZrO2/Y2O3 /CeO2 system outside the cubic region in order to achieve any technical effect correlative with the problem underlying the patent in suit.

Under these circumstances the Board can only conclude that the Appellant has by way of hindsight with knowledge of the invention, chosen compositions in ternary phase diagrams known from the prior art, but has not provided evidence as to why a person skilled in the art faced with the problem underlying the patent in suit would have done so.

7. It is accordingly the Boards view that the subject-matter of product claim 1 and dependent claim 2, as well as claim 3 relating to a method of producing a zirconia porcelain comprising each of the product features of claim 1 and dependent claims 4 and 5, would not have been obvious from either citation taken singly or in combination. Thus, the required inventive step is not lacking and the said claims satisfy the requirements of Article 56 EPC.