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Refractory fibers more than double strength of coating. Impact strengths of ceramic coatings increase with increasing whisker content. Silicon carbide whiskers clearly produce largest increase, and improvement grows even more with high-temperature sintering. Coating also improves thermal and mechanical properties of electromagnetic components, mirrors, furnace linings, and ceramic parts of advanced internal-combustion engines.
The mechanical properties of four advanced ceramic fabrics are measured at a temperature range of 23 C to 1200 C. The fabrics evaluated are silica, high-and low-boria content aluminoborosilicate, and silicon carbide. Properties studied include fabric break strengths from room temperature to 1200 C, and bending durability after temperature conditioning at 1200 C and 1400 C. The interaction of the fabric and ceramic insulation is also studied for shrinkage, appearance, bend resistance, and fabric-to-insulation bonding. Based on these tests, the low-boria content aluminoborosilicate fabric retains more strength and fabric durability than the other fabrics studied at high temperature.
A mixture containing glass 70 to 95 and BN or B4C powder (0.1-10 microns) 5 to 30 vol. % is used as a secondary pressure medium in hot isostatic pressing of ceramics. Thus, Pyrex beads were mixed with 15% vol. BN powder (average diameter 2 microns), fused at 1400 deg for 2 h, cooled, crushed, and put into a graphite crucible. A Si3N4 sintered body was embedded in the powder, heated in vacuum at 1200 deg for 2 h, treated in a hot isostatic press furnace at 1700 deg and 1000 atm. for 1 h, and cooled to give a Si3N4 ceramic. It was easily separated from the crucible.
In the hot isostatic pressing of ceramic materials in molten glass using an inert gas as a pressing medium, glass adhered to the sintered ceramics is heated to convert it to a porous glass and removed. Thus, Si3N4 powder was compacted at 5000 kg/sq cm, coated with a 0.5 mm thick BN, embedded in Pyrex glass in a graphite crucible, put inside a hot isostatic press containing Argon, hot pressed at 1750 C and 100 kg/sq cm; cooled, taken out from the crucible, heated at 1100 C for 30 minutes, cooled, and then glass adhered to the sintered body was removed.
Results of fundamental and focused research on the tribological properties of ceramics are discussed. The basic friction and wear characteristics are given for ceramics of interest for use in gas trubine, adiabatic diesel, and Stirling engine applications. The importance of metal oxides in ceramic/metal sliding combinations is illustrated. The formulation and tribological additives are described. Friction and wear date are given for carbide and oxide-based composite coatings for temperatures to at least 900 C.
The ability of scanning laser acoustic microscopy (SLAM) to characterize artificially seeded voids in sintered silicon nitride structural ceramic specimens was investigated. Using trigonometric relationships and Airy's diffraction theory, predictions of internal void depth and size were obtained from acoustic diffraction patterns produced by the voids. Agreement was observed between actual and predicted void depths. However, predicted void diameters were generally much greater than actual diameters. Precise diameter predictions are difficult to obtain due to measurement uncertainty and the limitations of 100 MHz SLAM applied to typical ceramic specimens.
The service lifetime of glass and ceramic materials can be expressed as a plot of time-to-failure versus applied stress whose plot is parametric in percent probability of failure. This type of plot is called a design diagram. Confidence interval estimates for such plots depend on the type of test that is used to generate the data, on assumptions made concerning the statistical distribution of the test results, and on the type of analysis used. This report outlines the development of design diagrams for glass and ceramic materials in engineering terms using static or dynamic fatigue tests, assuming either no particular statistical distribution of test results or a Weibull distribution and using either median value or homologous ratio analysis of the test results.
The mechanical properties of four advanced ceramic fabrics were measured at a temperature range of 23C to 1200C. The fabrics evaluated were silica, high and low-boria content aluminoborosilicate, and silicon carbide. Properties studied included fabric break strengths from room temperature to 1200C, and bending durability after temperature conditioning at 1200C and 1400C. The interaction of the fabric and ceramic insulation was also studied for shrinkage, appearance, bend resistance, and fabric-to-insulation bonding. Based on these tests, the low-boria content aluminoborosilicate fabric retained more strength and fabric durability than the other fabrics studied at high temperature.
Experimental and analytical studies were conducted to determine the acceptability of a ribbon-burner simulation of engine conditions for testing ceramic-lined turbine tip shrouds. The calculated values reveal that the ribbon burner establishes at least as harsh a thermal environment as is present at any time within the turbine shroud. Comparisons were made with ceramic components in a turboshaft engine.
Ceramic gas-path seals were fabricated and successfully operated over 1000 cycles from flight idle to maximum power in a small turboshaft engine. The seals were fabricated by plasma spraying zirconia over a NiCoCrAlX bond coat on the Haynes 25 substrate. Coolant-side substrate temperatures and related engine parameters were recorded. Post-test inspection revealed mudflat surface cracking with penetration to the ceramic bond-coat interface.
Optical-properties data for ceramic fabrics woven from silica, aluminoborosilicate, or silicon carbide yarns are presented. The normal hemispheric reflectances and transmittances measured at room temperature and over the spectral range of 250 to 2500 nm show a large dependence on fabric weight and a lesser dependence on fabric weave pattern. Reflectances of silicon carbide fabrics are lower by a factor of about 10 than those of silica and aluminoborosilicate. The reflectance and transmittance of silica and aluminoborosilicate fabrics decrease at wavelengths below 500 nm because of material absorption. A two-flux analytical model is used to derive spectral absorption and scattering coefficients for the silica and aluminoborosilicate fabrics from the measured reflectance and transmittance data, and comparisons are made with available data for other ceramics.
Chemical composition varies as function of depth. Technique developed to fabricate monolithic ceramic component by sintering laminated body made from thin sheets of green ceramic. Method allows discrete changes in chemistry effected across monolith. Component having one-dimensional variation in material composition fabricated in this manner.
The SCARE (Structural Ceramics Analysis and Reliability Evaluation) computer program on statistical fast fracture reliability analysis with quadratic elements for volume distributed imperfections is enhanced to include the use of linear finite elements and the capability of designing against concurrent surface flaw induced ceramic component failure. The SCARE code is presently coupled as a postprocessor to the MSC/NASTRAN general purpose, finite element analysis program. The improved version now includes the Weibull and Batdorf statistical failure theories for both surface and volume flaw based reliability analysis. The program uses the two-parameter Weibull fracture strength cumulative failure probability distribution model with the principle of independent action for poly-axial stress states, and Batdorf's shear-sensitive as well as shear-insensitive statistical theories. The shear-sensitive surface crack configurations include the Griffith crack and Griffith notch geometries, using the total critical coplanar strain energy release rate criterion to predict mixed-mode fracture. Weibull material parameters based on both surface and volume flaw induced fracture can also be calculated from modulus of rupture bar tests, using the least squares method with known specimen geometry and grouped fracture data. The statistical fast fracture theories for surface flaw induced failure, along with selected input and output formats and options, are summarized. An example problem to demonstrate various features of the program is included.
Results of fundamental and focused research on the tribological properties of ceramics are discussed. The basic friction and wear characteristics are given for ceramics of interest for use in gas turbine, adiabatic diesel, and Stirling engine applications. The importance of metal oxides in ceramic/metal sliding combinations is illustrated. The formulation and tribological additives are described. Friction and wear data are given for carbide and oxide-based composite coatings for temperatures to at least 900 C.
The Weibull statistical theory of fracture was applied to thermal shock of ceramics in the water quench test. Transient thermal stresses and probability of failure were calculated for a cylindrical specimen cooled by convection. The convective heat transfer coefficient was calibrated using the time to failure which was measured with an acoustic emission technique. Theoretical failure probability distributions as a function of time and quench temperature compare favorably with experimental results for three high-alumina ceramics and a glass.
The SCARE (Structural Ceramics Analysis and Reliability Evaluation) computer program on statistical fast fracture reliability analysis with quadratic elements for volume distributed imperfections is enhanced to include the use of linear finite elements and the capability of designing against concurrent surface flaw induced ceramic component failure. The SCARE code is presently coupled as a postprocessor to the MSC/NASTRAN general purpose, finite element analysis program. The improved version now includes the Weibull and Batdorf statistical failure theories for both surface and volume flaw based reliability analysis. The program uses the two-parameter Weibull fracture strength cumulative failure probability distribution model with the principle of independent action for poly-axial stress states, and Batdorf's shear-sensitive as well as shear-insensitive statistical theories. The shear-sensitive surface crack configurations include the Griffith crack and Griffith notch geometries, using the total critical coplanar strain energy release rate criterion to predict mixed-mode fracture. Weibull material parameters based on both surface and volume flaw induced fracture can also be calculated from modulus of rupture bar tests, using the least squares method with known specimen geometry and grouped fracture data. The statistical fast fracture theories for surface flaw induced failure, along with selected input and output formats and options, are summarized. An example problem to demonstrate various features of the program is included.
The ability of scanning laser acoustic microscopy (SLAM) to characterize artificially seeded voids in sintered silicon nitride structural ceramic specimens was investigated. Using trigonometric relationships and Airy's diffraction theory, predictions of internal void depth and size were obtained from acoustic diffraction patterns produced by the voids. Agreement was observed between actual and predicted void depths. However, predicted void diameters were generally much greater than actual diameters. Precise diameter predictions are difficult to obtain due to measurement uncertainty and the limitations of 100 MHz SLAM applied to typical ceramic specimens.
Proposed apparatus for inspecting prefired ceramic materials detects minute flaws that might escape ordinary visual inspections. Method detects flaws and marks locations. Intended for such thin ceramic parts as insulation in capacitors and some radio-frequency filters.