TiN coating of alumina platelets and mullite fibres using a precipitated TiCl_4.4NH_3 precursor for incorporation into ceramic metal composites

摘要

The incorporation of alumina into TiC-TiN-Mo_2C-Ni cermets should enhance their oxidationand corrosion resistance at high temperatures. Onedrawback in the realisation of these composites is thepoor wetting of alumina by liquid nickel during theliquid phase sintering [l]. This work investigates thecoating of alumina or mullite particles with TiN toenhance their wetting properties enabling theproduction of such composites. The coating of mullitefibres and alumina platelets was carried out by theprecipitation of TiCl_4.4NH_3 followed by a thermaltreatment. Two methods were investigated to transformthe precipitated precursor into TiN. 1) Transformationof the precursor into an oxide followed by acarbothermal reduction, 2) Direct transformation ofthe precursor into TiN by thermal treatment in anitrogen atmosphere. The second direct route having theadvantage of being a single step rather than the twosteps of oxidation and carbothermal reduction. Thecoatings and transformation steps were characterisedusing SEM, XRD, XPS, TGA/DTA, elemental and microprobeanalysis. During the transformation into an oxide inthe first route the coating on the alumina plateletsshowed a tendency to sinter and coalesce leading to poorcoverage. Two dopants were introduced, either Si or Ta,to reduce this effect which was partially successfulfor the Ta. The carbothermal reduction gave a completetransformation of the coating into TiN. TiC_xN_(1-x)coatings were also produced using this carbothermalroute. The direct nitridation of the precursor, thesecond route, showed a material loss (20-60% of the Ti)due to the volatility of the precursor but thetransformation to TiN was successful. The TiN coatingobtained by the two methods were nearly continuous onthe mullite fibres but the platelets were onlypartially coated. From XPS studies the coverage on bothparticles was better via direct nitridation than viathe carbothermal route, despite the lowerconcentration of Ti on the platelets with the directroute. Preliminary tests using the coated platelets inthe fabrication of TiC-TiN-Mo_2C-Ni-Al_2O_3 cermetsshowed a small improvement in the sintered densities.