초고속 회전체의 내구성향상을 위한 Co-alloy(T800)의 초고속 용사코팅

The traditional high wear resistant coatings such as hard chrome plating and ceramic coatings have been replacing progressively by other coating methods such as HVOF thermal spray coating because of the environmental pollution of very toxic hex-Cr known as carcinogen having toxicity greater than arsenic and cadmium by chrome plating and chrome plated products and the brittleness of ceramic coatings(6, 7). Co-alloy(T800) micron size powder was coated on Inconel 718 by HVOF thermal spraying for the studies of the improvement of durability of high speed air bearing spindle. Coatings were prepared by Taguchi program for the parameters of spray distance, flow rates of hydrogen and oxygen and powder feed rate. Optimal coating process was determined by the studies of coating properties such as micro-structure, porosity, surface roughness and micro hardness. Friction and wear behaviors of coatings were investigated by sliding wear test with counter sliding stainless steel 304 ball at room temperature and at elevated temperature of 1000°F(538℃). At room temperature wear debris of the coating were roughly reduced to 15 times less and friction coefficients were decreased to more than a half compared with the surface of non-coated parent material. At high temperature of 538℃ wear debris and friction coefficients of the coatings were drastically decreased compared with non-coated surface of parent material. These drastic decreases of both wear debris and friction coefficient of the coatings compared with those of non-coated surface of parent material shows that Co-alloy(T800) coating is highly recommendable for the durability improvement surface coating of high speed air-bearing spindle. At high temperature wear traces and friction coefficients of both coating and non-coating were drastically reduced compared with those of room temperature since the oxides such as CoO, CO₃O₄, MoO₂, MoO₃, and etc were formed easily on the surface, and the brittle oxide phases were attrited by the reciprocating sliding wear according to the complicated mixed mechanisms such as oxidative wear by direct reaction with oxygen, abrasion by scratching or gouging at the asperities by the sliding ball, slurry erosion by the mixture of solid particles and small drops of the melt of the attrited particles, cavitation by the relative motions among the coating, sliding ball and melt, and corrosive wear by the corrosion environment induced by the frictional heat and local high temperature(8, 17). These oxide particles and the melt play role as lubricant and reduce the wear and friction coefficient. This also shows that Co-alloy(T800) coating is highly recommendable for the durability improvement surface coating on the surface vulnerable to frictional heat such as high speed air-bearing spindles.