REPORT 99 CL. o Q_ ADVISORY GROUP FOR AERONAUTICAL RESEARCH AND DEVELOPMENT REPORT 99 CERMETS AS POTENTIAL MATERIALS FOR HIGH-TEMPERATURE SERVICE by O. A. SANDVEN APRIL 1957 NORTH ATLANTIC TREATY ORGANIZATION PALAIS DE CHAILLOT. PARIS 16 REPORT 99 NORTH ATLANTIC TREATY ORGANIZATION ADVISORY GROUP FOR AERONAUTICAL RESEARCH AND DEVELOPMENT CERMETS AS POTENTIAL MATERIALS FOR HIGH-TEMPERATURE SERVICE by 0. A. Sandven This Report was presented at the Fifth Meeting of the Structures and Materials Panel, held from 24th to 27th April 1957, in Oslo, Norway SUMMARY A review is given of the chemical, physical and mechanical properties of the most important and promising Hard Metals and Cermet systems, with special attention to the creep resistance and ductility. Some experimental results on the system NbC-TiC Ni are reported. SOMMAIRE Revue des proprietes chlmiques, physiques et mecaniques des metaux durs et de» cermets les plus importants et les plus prometteurs, en considerant plus particullerement leur resistance au fluage et leur ductilite. Presentation de quelques resultats d'essais obtenus avec le systeme NbC-TiC Ni. 666,762 2a9g2:3e3c5c ii CONTENTS page SUMMARY i i LIST OF TABLES iv LIST OF FIGURES iv 1. INTRODUCTION 1 2. HARD METALS AND THEIR PROPERTIES 1 2.1 General 1 2.2 Carbides 2 2.3 Borides, Nitrides and Silicides 2 3. METAL BONDED HARD METALS (CERMETS) 3 3.1 Gene ra1 3 3.2 Strength at Elevated Temperatures 3 3.3 Impact Strength and Thermal Shock Resistance 4 3.4 Oxidation Resistance 4 4. CONCLUSIONS 5 REFERENCES 6 TABLES 7 FIGURES 9 DISTRIBUTION iii LIST OF TABLES page TABLE I - Properties of Carbides 7 TABLE II - Properties of Nitrides 7 TABLE III - Properties of Borides 8 TABLE IV - Properties of Sillcides 8 LIST OF FIGURES Fig. 1 Mutual solid-solubility in binary carbide systems 9 Pig. 2 Mutual solid-solubility in binary nitride systems 10 Fig. 3 Mutual solid-solubility in binary nitride-carbide systems 11 Pig. 4 Mutual solid-solubility in binary disilicide systems 12 Fig. 5 Isothermic sections of ternary disilicide systems. 1300°C 13 Fig. 6 100 hr strength of TiC cermets 15 Pig. 7 100 hr strength of TiC-base cermets 16 Fig. 8 Stress-rupture time (1% strain). TiC-base cermets at 950°C 17 Fig. 9 Stress-rupture time. 980°C 18 Fig.10 Oxidation of WZ-cermets. 1100°C 19 Pig.11 Oxidation of cermet and superalloy 20 Pig.12 Oxidation of TlC-Cr3C2. 1 hr 21 Fig.13 Oxidation of WZ-cermets 22 Fig.14 Oxidation of TiC-base cermets. 1100°C 23 Fig.15 Oxidation of 70 (TiNb)C, 24 Ni, 6 Cr cermets 24 iv CERMETS AS POTENTIAL MATERIALS FOR HIGH-TEMPERATURE SERVICE 0. A. Sandven* 1 . INTRODUCTION Technical development, especially on the field of jet propulsion, has undoubtedly been delayed by the lack of suitable heat-resistant construction materials. The alloys at present available for such purposes, that is Ni-base and Co-base alloys, lose their strength so rapidly with increasing temperature that they cannot be used with any success above 800 - 900°C. and the development of other and more suitable alloys for high-temperature purposes, is therefore a problem of primary importance In modern metallurgy. Considerable research work has been carried out recently to solve this problem and, among the various types of materials which seem to be of potential use, the cemented Hard Metals, or cermets, play an important part. Many reports dealing with cermets, specially TiC-base cermets, have been published, and it has been shown that it is possible to obtain cermet materials with supreme high-temperature strength and oxidation resistance. Unfortunately, however, all the cermet materials known so far also have undesirable properties, which makes them unfit for constructional use, except for some special and very limited purposes. Among these undesired properties, the lack of impact strength is possibly the most difficult and serious to overcome, although high cost, difficult production and machinability also have to be considered. 2. HARD METALS AND THEIR PROPERTIES 2.1 General The metallic hard metals, that is, the carbides, borides, nitrides and sllicldes of the transition metals in the (d) - (f) group, have some common properties. The most important of these are:- (a) Very high melting point (b) Very high hardness (c) High chemical stability (d) Metallic character (metallic lustre, conductors of heat and electricity) (e) They have mostly a interstitial-solid-solution structure (f) They have a very high modulus of elasticity. Many of the (d) - (f) group metals form more than one chemical compound with each of the non-metals C. N, B and Si (ex WC and W2C), but it is mostly the mono-carbides and -nitrides, diborides and disillcides which have been studied. *Sivilingenior, Research Metallurgist, Norwegian Defence Research Establishment 2.2 Carbides The carbides are the best known hard metals. With the exception of WC, all the metallic monocarbldes have an P.P.C, - structure. They have melting points above 2000°C, high modulus of elasticity, good oxidation resistance and little or no ductility. Table I gives some properties of the most important metallic carbides, and also of the two carbides B2C and SIC. From the present point of view, the carbides TiC, TaC, NbC and Cr3C2 are of primary importance. The first three are completely soluble in each other in the solid state, and work on the quasi-binary systems TiC-TaC, TiC-NbC and NbC-TaC have been published (see Reference 2. pp. 165-196). Unfortunately, however, in cermet development, most work deals with Ta(Nb)C in TiC (Ref.3). This gives some information of the Influence of the pure carbides NbC and TaC on TiC, since NbC and TaC seem to be very similar, but the exact nature of this influence cannot be found in this way. Workers using the pure carbides4 have found that NbC-addition to TiC-cermets increases the creep resistance more than a similar TaC-addition. Also the oxidation resistance of TiC will be increased considerably by the addition of TaC, NbC and Cr3C2 (Ref. 2, pp. 659-667 and Ref.5). It is of interest to notice that the carbides TaC and NbC have some ductility1. This does not mean that they can be deformed plastically to any great degree, but merely that they can undergo some plastic deformation without failure. As might be expected, the mutual solid-solubility is very great in quasi-binary carbide-carbide systems (see Figure 1 and Reference 2, pp. 59-157, 326). All the cubic carbides are completely soluble in each other in the solid state, with the exception of the pair VC-ZrC. This system has very narrow fields of terminal solid- solutions, possibly due to the rather large difference between the lattice para­ meters of VC and ZrC. Some of the carbides, or carbide-systems, will possibly not be of any potential value for the development of high-temperature materials. This is so with HfC, which is not available in any great quantity. The high densities of TaC and WC will possibly restrict their use, save for rather small additions to other carbides, and the extremely hard and brittle non-metallic carbides B14C and SIC may possibly also be of little value. However, the remaining carbides, in some combination with each other, or in a pure form, may be able to form the basic constituents in materials suitable for high-temperature use; although, in spite of considerable research which has been undertaken on these hard metals, their nature and properties are not yet well understood. 2.3 Borides, Nitrides and Silicides The properties of the metallic borides, nitrides and silicides (d) - (f) group transition metal) are not quite so well known as the properties of the carbides, but they are in many respects very similar to the carbides. Again the common properties are high hardness, high melting point and great chemical stability, although the crystal structures are somewhat more complex. The mutual solid-solubility of nitride-nitride systems (Ref. 2, pp. 212-250) are shown in Figure 2. Perfect solid-solubility is found between cubic nitrides, except in systems VN-ZrN and VN-HfN. The hexagonal structure of TaN, restrict the solid- solution formation in systems between this component and one of the cubic nitrides. Figure 3 gives the solubility on some nitride-carbide systems (Ref. 2, pp. 212- 250). Because of similarity in structure, many of these systems have complete solid- solution formation, but ZrC, is not stable in the presence of N at very high temperatures. Relatively little is known about the boride-boride systems (Ref. 2, pp. 259-292) but, by analogy with the carbides and nitrides, it is to be expected that a large mutual solid-solubility between the diborides of Ti, Zr, Hf and V, Mb and Ta will exist, because they all have the same structure (ACB2-type hexagonal). In boride-carbide systems, no solid-solubility formation can be expected, because B cannot substitute C in the metal lattice. The diborides are mostly stable In the presence of carbon (Ref.2, pp. 259-292) but the diborides of V, Nb and Ta decompose during melting (in the presence of carbon) to Monoborides + Bor. It is interesting to note that the _d is 1lie ides TaSi2, MoSi2 and NbSi2 have some ductility. TaSi2 (which is the most ductile one) and MoSi2 have also excellent oxidation resistance. The mutual solid-solubility in disilicide systems is fairly well known (Ref. 2, pp. 301-325 and Refs. 6,7) (Fig.4). The variation in crystal structure among the sili­ cides naturally restricts the solid-solution formation, but the hexagonal slllcldes TaSi2, NbSi2 and VS12 are completely soluble in each other in the solid state.