Tham khảo tài liệu 'materials handbook 15th ed - g. brady_ h. clauser_ j. vaccari (mcgraw-hill_ 2011) ww part 4', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 210 CERAMICS thermal conductivity and differences between tensile and compressive strengths. Unlike metals ceramics have relatively few free electrons and therefore are essentially nonconductive and considered to be dielectric. In general dielectrical strengths which range between 200 and 350 V mil X 106 and X 106 V m are lower than those of plastics. Electrical resistivity of many ceramics decreases rather than increases with an increase in impurities and is markedly affected by temperature. Practically all ceramic materials have excellent chemical resistance being relatively inert to all chemicals except hydrofluoric acid and to some extent hot caustic solutions. Organic solvents do not affect them. Their high surface hardness tends to prevent breakdown by abrasion thereby retarding chemical attack. All technical ceramics will withstand prolonged heating at a minimum of 1830 F 999 C . Therefore atmospheres gases and chemicals cannot penetrate the material surface and produce internal reactions which normally are accelerated by heat. Aluminum-ceramic coatings are used to protect aircraft-turbine and other turbomachinery parts from corrosion and heat at temperatures to 2000 F 1093 C and greater. For compressor applications in ground-based turbines aluminum-filled chromate-phosphate coatings sealed with a ceramic topcoat have more than doubled service life. Aluminum-ceramic coatings are also alternatives to cadmium plating of fasteners and other products and used for galvanic protection of dissimilar materials. Nickel-ceramic coatings with silicon carbide or silicon carbide and phosphorus added to the nickel matrix for hardness and hexagonal boron nitride or silicon nitride for lubricity are used in Japan on cylinder bores and pistons of outboardmarine motorcycle and snowmobile engines to increase wear resistance. Paintable ceramic coatings a specialty of Zyp Coatings Inc. combine corrosion resistance with heat resistance to 2000 F 1093 C . Piezoelectric .
