Chemical Vapor Deposition of Erosion‐Resistant TiB2 Coatings

Abstract

Erosion‐ and wear‐resistant coatings are needed for numerous applications, including valve and pump components for coal liquefaction and gasification plants. Titanium diboride is unusually hard and stable and offers considerable promise for use in highly erosive and corrosive environments. We deposited coatings by hydrogen reduction of and . Our objective was to correlate process variables with coating structure and properties, with emphasis on obtaining coatings having high erosion resistance. We varied deposition temperatures from 750° to 1050°C and the and flow rates. Commercial cemented carbides and experimental nickel‐bonded , were used as substrates. After structural characterization, the resistance of the deposited coatings to erosion was determined by a hot coal‐oil slurry impingement test. The deposition rate was very temperature dependent, ranging from 0.1 to 2.9 μm/min between 750° and 950°C. The surface of the coatings showed nodules increasing in size with temperature. The coatings were dense and adherent. The grain size varied with deposition conditions. Often, the grains were too small to resolve optically, but transmission electron microscopy showed the grain size to be very small (2–300 nm) and to increase with distance from the substrate. Electron microscopy and x‐ray diffraction showed only single‐phase , and ion microprobe analysis revealed a constant Ti:B ratio across the coating thickness. Energy‐dispersive fluorescence analyses showed more chlorine in coatings deposited at 800°C than in coatings deposited at higher temperatures. The Knoop microhardnesses of coatings deposited at 800° and 900°C were 15 and 33 GPa, respectively. Coatings deposited at 850°C or below eroded extensively (up to 30 μm deep craters during a 1h test), while those deposited at 900°C showed very little or no erosion (0–3 μm).