Structural evaluation of phosphate bonded ceramic composite materials from nondestructive ultrasonic velocity and attenuation measurements

The fabrication of a ceramic consisting of a matrix of newberyite and aluminium orthophospate filled with alumina and lesser amounts of carbon and glass fibre, is described. This material, whilst combining excellent insulation properties with forgiving fracture, is easy to produce by moulding without sintering. The ceramic has been characterized by a number of complementary ultrasonic techniques in the frequency range 24 kHz to 5 MHz,Dynamic elastic moduli measurements have been found to agree well with elastic constants calculated theoretically by treating the matrix and filler as end members of a two-phase material whose properties obey the lower Hashin and Shtrikman bound. In addition the bulk modulus of the matrix computed theoretically from crystallographic data and making an allowance for porosity agreed closely with the experimental modulus. Thus ultrasound velocity (moduli) measurements combined with theory can be used for non-destructive monitoring and quality control of the ceramic composition which is subject to variation with the parameters governing the chemical reaction during preparation. The theoretical bulk modulus of the ideal (pore-free) matrix is 10.4 GPa. This matrix modulus is far less than that of the moduli of the constituent oxides in the starting mixture. The reason for this is the large expansion in the sizes of closed rings of cation-oxygen network bonds that takes place in the reaction, rather than structural weakening (breaking of rings of network bonds) by hydration.The frequency dependence of ultrasonic attenuation has been used to identify scattering regimes and thus determine the dimensions of the major scattering particles. Grain sizes determined ultrasonically for the three compositions showed excellent agreement with values determined by optical microscopy.The high frequency dependent absorption and scattering in this material, mean that good ultrasound propagation is obtained only at low frequencies. The lowest frequency at which the ultrasonic propagation and properties are dependent on the material structure alone, i.e. independent of sample size, has been established to be 2 MHz with conveniently sized test pieces of dimensions 1.5×1.5×6 cm³. Previous address: Department of Physics, Brunel University, Kingston Lane, Uxbridge, Middlesex, UK. Previous address: Department of Physics, Brunel University. Previous address: Thorn EMI Central Research Laboratories, Dawley Road, Hayes, Middlesex, the Mechanics Group, Department of Engineering, University of Reading. Now at Fison's Scientific Instruments, Manor Industrial Estate, Gatwick Road, Crawley, Surrey. UK.