This invention relates to the production of titanium diboride and is directed to an improved highly reactive, non-pyrophoric, sub-micron amorphous and/or crystalline titanium diboride powder, and coatings, layers, components, and monolithic shapes made therefrom.
A variety of methods have been developed for producing metal boride powders such as titanium diboride, as follows:
1. Direct combination of the metal with boron or a metal hydride and boron.
2. Hydrogen reaction of a boron halide with a metal source such as an oxide.
3. Electrolysis of a melt consisting of a metal oxide, boron oxide, and an alkali flux.
4. Reduction of a metal oxide with boron (borothermic) or co-reduction with oxide by carbon (carbothermic).
5. Reduction of oxides by metals.
6. Arc-plasma reaction from vapor reactants.
Methods 1-3 are used mainly in laboratory experimentation. Method 4 has been employed commercially but the borothermic method requires large amounts of boron over what the final product contains and is thus too wasteful for high production levels. The carbothermic method, however, is the one generally used to commercially produce titanium diboride. In this method, the raw materials are heated to a temperature in the range of 1800.degree.-2000.degree. C. until titanium diboride is formed. The powdered titanium diboride ultimately recovered requires forming temperatures in about this same range. Method 5 has been attempted but found to produce contaminants of other borides as well as borates and titanates, and has not been considered a commercially viable method. Method 6 produces pyrophoric titanium diboride from expensive reagents.
The principle disadvantages of the above methods are, for 1, expensive raw materials and the likelihood of formation of potentially explosive boranes; for 2-4, expensive raw materials and reaction vessels; for 5, production of contaminant borides, borates and titanates; and for 6, expensive raw materials, reaction vessels and formation of pyrophoric titanium diboride.
The carbothermic method, method 4, is currently being used commercially to produce relatively large grained, crystalline titanium diboride. High temperatures (1800.degree.-2000.degree. C.) are required in the processing; and in order to obtain the product in sub-micron size, final grinding, a source of contamination, is also required. The resultant powdered, sub-micron titanium diboride is crystalline (hexagonal), low in surface area (6 m.sup.2 /gm BET), and is difficult to sinter and form into dense shapes. The powder must be vacuum sintered at 2200.degree. C. or hot pressed under high pressure at 1800.degree.-2200.degree. C. Even at carefully controlled processing conditions, micro-stresses are caused by grain growth and thermal expansion of individual crystallites by different amounts in the a and c directions.
Another titanium diboride material disclosed by Byrnestad, U.S. Pat. No. 4,503,021, is an ultra-fine material with a particle size of less than 1/10 of a micron. However, this material has a relatively small surface area and is highly pyrophoric. Other disadvantages of the Byrnestad material include the need for relatively high temperature and pressure in forming dense shapes by hot pressing or sintering. Additionally, the dense shape formed from the Brynestad material has a relatively large grain size, a relatively low modulus of rupture and a relatively low elastic modulus.
A Japanese titanium diboride, Japanese Pat. No. 089363, is a non-pyrophoric material having a grain size of less than 2.5 microns; however, the Japanese material is produced by using an excess of MgO in the raw materials to control the violent reaction. The resulting MgO/TiB.sub.2 powder is purified by using chloride based leach solutions which can result in a chlorine-contaminated final product. Additionally, the raw materials must be ball milled together prior to reaction, a process which along with adding impurities from the milling process adds extra time and expense. The resultant TiB.sub.2 must be ultra-centrifuged to remove the supernatant liquid, another processing step adding to the cost. A further disadvantage of the Japanese product is the need to form the product in an inert atmosphere (argon), thus also adding expense.