1. Field of the Invention
This invention relates to a dense silicon nitride sintered body. More particularly, this invention relates to a high density silicon nitride material having excellent fracture toughness and excellent strength, and to a method of manufacturing the same.
2. Description of the Prior Art
Sintered silicon nitride ceramics are well-known for their excellent strength at temperature in excess of 1000.degree. C. However, at temperature greater than 1200.degree. C. for the advanced turbine engine applications, few silicon nitride ceramics meet the strength requirement. Furthermore, conventional silicon nitride ceramics have fracture toughness typically ranging from 4 to 6 MPa.multidot.m.sup.0.5, such low toughness makes them susceptible to significant strength degradation from the damage introduced during engine operation. It would be desirable to provide a silicon nitride having both high fracture toughness, hence strong resistance to damage, and high strength both at room and elevated temperature. Moreover, it would be most desirable to have a silicon nitride material with this combination of excellent properties which can easily be formed into near net shape parts of complex geometry.
Sintering silicon nitride requires sintering aids which form grain boundary phases. Rare earth oxides are effective sintering aids and form refractory grain boundary phases yielding silicon nitride ceramics with good high temperature properties. However, they often require high sintering temperature and/or the application of external pressure for complete densification.
Hot pressing generally produces silicon nitride ceramics with excellent strength properties. U.S. Pat. No. 4,234,343 to Anderson discloses that hot-pressed silicon nitride containing different rare earth oxides as sintering aids can have 250 MPa to 550 MPa strength at 1400.degree. C. with smaller rare earth element resulting in higher 1400.degree. C. strength. Ueno and Toibana report in Yogyo-Kyokai-Shi, vol. 91, 409-414 (1983) that hot-pressed silicon nitride containing yttria (Y.sub.2 O.sub.3) in combination with other rare earth oxides exhibits strength of over 600 MPa at 1300.degree. C. Despite these property advantages, it is well known in the field that the process of hot pressing has limited value in the production of structural ceramics because of its shape and size limitations
Hot isostatic pressing has the same advantages as hot pressing but without the shape and size limitations. U.S. Pat. No. 4,904,624 to Yeckley teaches the fabrication of silicon nitride parts containing rare earth sintering aid with flexural strength in excess of 525 MPa at 1370.degree. C. using glass-encapsulated hot isostatic pressing. However, the fracture toughness of this Si.sub.3 N.sub.4 is only 4 to 5 MPa.multidot.m.sup.0.5. Similarly, U.S. Pat. No. 4,870,036 to Yeh teaches how to fabricate silicon nitride ceramics containing yttria and strontium compound having flexural strength greater than 465 MPa at 1375.degree. C. using hot isostatic pressing, but the fracture toughness of this Si.sub.3 N.sub.4 is 5 to 6 MPa.multidot.m.sup.0.5. Thus, although hot isostatic pressing can produce silicon nitride ceramics with excellent strength, the fracture toughness of such material is low.
Gas pressure sintering is a manufacturing process for silicon nitride employing moderate nitrogen pressure during high temperature firing. It can be used to fabricate refractory silicon nitride parts without shape and size limitations. U.S. Pat. No. 4,628,039 to Mizutani et al. describes using gas pressure sintering to fabricate silicon nitride ceramics having excellent four-point bending strength at 1300.degree. C. Said silicon nitride ceramics contain sintering aids consisting of oxides of two rare earth elements having ionic radii greater and smaller than 0.97 .ANG. respectively, and other minor additives such as oxides of elements from Group IIa of the Periodic Table. U.S. Pat. No. 4,795,724 to Soma et al. describes gas pressure sintered silicon nitride containing at least two kinds of sintering aids selected from Y, Er, Tm, Yb, and Lu having 1400.degree. C. flexural strength of at least 500 MPa; an example given in this patent shows that a gas-pressure-sintered silicon nitride ceramic containing Y.sub.2 O.sub.3 and La.sub.2 O.sub.3 has a strength of only 230 MPa at 1400.degree. C. No information concerning fracture toughness was provided in the above-identified inventions.
It has been reported that silicon nitride containing 10 to 50% by volume silicon carbide, according to U.S. Pat. No. 3,890,250, and up to 40% by volume silicon carbide, according to U.S. Pat. No. 4,184,882, has improved strength at 1400.degree. C.; the ceramics taught by those patentees were prepared by hot pressing and their fracture toughness was not reported. U.S. Pat. No. 4,800,182 to Izaki et al. discloses a hot-pressed silicon nitride/silicon carbide composite, with 5 to 30 wt % of silicon carbide, having three-point bending strength of at least 930 MPa at room n temperature and fracture toughness of 5.3 to 7 MPa m.sup.0.5 depending on the silicon carbide content. U.S. Pat. No. 4,814,301 to Steinmann and Lip discloses the fabrication of a sintered silicon nitride using crystalline silicates and metal carbides with high retained strength at 1200.degree. C. The strength of those silicon nitride ceramics at 1375.degree. C. will not be high since silicates containing Na, Ca, Mg, Al, and Fe, etc. are used.