Nitride ceramics

Ceramic nitrides have many of the same properties as carbides and nitrides are also difficult to fabricate, especially in the pure form, due to strong covalent bonding. Sihcon nitride and boron nitride are the primary materials in the family of nitrides to be developed for engineering applications. ... 

The following example shows how the surface reaction rate is adjusted to improve the adhesion of a coating of a ceramic nitride to two different substrates ... 

Induction-hardened 0.4% C steel Sluny/sinler fanned ceramic Nitrided 316 stainless steel... 

The AET was used at standard tests of numerous structural materials, above all steels and cast iron, prepared are ceramic samples. Part of tested samples had qjecial sur ce layer treatments by laser, plasma nitridation and similar. Effect of special surface treatment the authors published already earlier [5,6]. In this contribution are summed up typical courses of basic dependencies, measured by the AET at contact loading. 

Evans R, Smith I, Munz W D, Williams K J P and Yanwood J 1996 Raman microscopic studies of ceramic coatings based on titanium aluminum nitride ICORS 96 XVth Int. Conf. on Raman Spectroscopy ed S A Asher and P B Stein (New York Wiley) pp 596-7... 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

SiHcon nitride (see Nitrides) is a key material for stmctural ceramic appHcations in environments of high mechanical and thermal stress such as in vehicular propulsion engines. Properties which make this material uniquely suitable are high mechanical strength at room and elevated temperatures, good oxidation and creep resistance at high temperatures, high thermal shock resistance, exceUent abrasion and corrosion resistance, low density, and, consequently, a low moment of inertia. Additionally, siHcon nitride is made from abundant raw materials. 

Properties. Properties of stmctural siHcon nitride ceramics are given in Table 2. These values represent available, weU-tested materials. However, test methodology and the quaHty of the specimens, particularly their surface finish, can affect the measured values. Another important material property is tensile strength. Values obtained on Norton s NT154 material are 750 MPa at RT, 500 MPa at 1200°C, and 350 MPa (50,000 psi) at 1400°C (62). 

As noted, the oxidation resistance of silicon nitride ceramics depends on the type and concentration of the sintering aids. In materials designed for high temperature appHcations the specific weight gain resulting from oxidation upon a 500-h air exposure at 1200°C and 1350°C is about 1—2 g/m and 2—4 g/m, respectively. The kinetics of the oxidation process have been iavestigated (63,64) as has the corrosion resistance (65). Corrosion resistance is also dependent on material formulation and density. 

Exposure limits for siHcon carbide and powders of zirconium compounds (including zirconium dioxide) have been estabHshed by ACGIH. TLV—TWA s are 10 mg/m and 5 mg/m, respectively. OSHA guidelines for zirconium compounds call for a PEL of 5 mg/m. There are no exposure limits for siHcon nitride powder, but pmdent practice suggests a TLV—TWA of 0.1 mg/m. The soHd ceramics present no apparent health hazard. In machining such ceramics, however, care should be taken to prevent inhalation of respirable particles in amounts in excess of estabHshed limits. Disposal should be in approved landfills the materials are inert and should pose no danger to the environment. 

A more extensive comparison of many potential turbine blade materials is available (67). The refractory metals and a ceramic, sHicon nitride, provide a much higher value of 100 h stress—mpture life, normalised by density, than any of the cobalt- or nickel-base aHoys. Several intermetaHics and intermetaUic matrix composites, eg, aHoyed Nb Al and MoSi —SiC composites, also show very high creep resistance at 1100°C (68). Nevertheless, the superaHoys are expected to continue to dominate high temperature aHoy technology for some time. 

Metal powder—glass powder—binder mixtures are used to apply conductive (or resistive) coatings to ceramics or metals, especially for printed circuits and electronics parts on ceramic substrates, such as multichip modules. Multiple layers of aluminum nitride [24304-00-5] AIN, or aluminay ceramic are fused with copper sheet and other metals in powdered form. The mixtures are appHed as a paste, paint, or slurry, then fired to fuse the metal and glass to the surface while burning off the binder. Copper, palladium, gold, silver, and many alloys are commonly used. 

Sihcon nitride occurs in two forms, a-Si N and P-Si N. Pure Si N is white, but the colors of commercial materials may be tan, gray, or black because of residual siUcon or impurities. Si N may be prepared by nitriding siUcon powder at 1200—1400°C or, for extremely fine-grained Si N, by the reaction of SiCl or SiH and N2 or NH (see also Advanced ceramics). 

Materials used for substrates can be broadly classified into ceramics and metals. Gommonly used ceramics, ie, alumina, aluminum nitride, and beryUia, can be easily incorporated into a hermetic package, ie, a package permanently sealed by fusion or soldering to prevent the transmission of moisture, air, and other gases. 

Although beryllium oxide [1304-56-9] is in many ways superior to most commonly used alumina-based ceramics, the principal drawback of beryUia-based ceramics is their toxicity thus they should be handled with care. The thermal conductivity of beryUia is roughly about 10 times that of commonly used alumina-based materials (5). BeryUia [1304-56-9] has a lower dielectric constant, a lower coefficient of thermal expansion, and slightly less strength than alumina. Aluminum nitride materials have begun to appear as alternatives to beryUia. Aluminum nitride [24304-00-5] has a thermal conductivity comparable to that of beryUia, but deteriorates less with temperature the thermal conductivity of aluminum nitride can, theoreticaUy, be raised to over 300 W/(m-K) (6). The dielectric constant of aluminum nitride is comparable to that of alumina, but the coefficient of thermal expansion is lower. 

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