C. Reynaud et al. International Journal of Refractory Metals d Hard Materials 19(2001)425-435 ormulation must be optimised for 40 vol% of PFA polished microstructure. Polished surfaces were etched incorporated in the slurry during 40 min in a plasma apparatus(Plasmod March) Rheological characterisations of the slurry have shown a under an atmosphere composed of CF4 +8 vol%O shear-thinning behaviour. Furthermore, slurries con- Same parameters listed above of 1200 ceramic grains taining polyamide powders as PFA exhibit higher vis- have been measured osity than the slurries using corn starch. Stronger chemical interactions between polyamide particles and tape casting organics may explain this result Then the slurries were tape cast onto a Mylar film by 3. Results and discussion the way of a moving double blade device at speed be- tween 0.17 and 0.26 m/min on a laboratory tape casting 3.1. PFA thermal degradation bench(Cerlim Equipement, Limoges-France). Strong and soft tapes with smooth surfaces, uniform thickness A chemical analysis of the gas resulting from the between 100 and 150 um were obtained. thermal degradation of the PFA has been investigated Disks(diameter 20 mm) and sheets (50 x 50 mm by using a Fourier Transformation-Infra Red (FTIR were punched in green tapes. Stack of two types com- spectrometer(Equinox 55, BRUKER) coupled to a posed of 40-45 layers (total thickness 4-5 mm)were thermogravimetric analyzer (TG 209, NETZSCH). Two gas atmospheres(Oz, Ar) have been used to study the monolithic blocks of same stacked layers, effect of the oxidant character of the atmosphere on the symmetric multilayered materials constituted by an thermal degradation. Indeed, during the burn-out under alternative stacking of dense and porous layers air, the gas atmosphere near the sample surface may be After stacking, specimens were pressed under 60 MPa mainly composed of air whereas the atmosphere cor has bee perature near 65 C. The burnout of organics position inside the sample is unknown due to the inward performed in air by heating with an heating diffusion of oxygen and nitrogen atoms and to the rate of o i oCImin between 20 and 550 C with inter- outward diffusion of the thermal decomposition prod mediate dwells according to thermogravimetric analysis ucts in the opposite way. An inert atmosphere may be of laminar materials assumed in the core of the sample Sintering of the specimens was conducted in a Polyamide powders appears more sensitive to the oxidant character of the atmosphere than corn starch nosphere during Ih at a temperature of 1950 %C. The (Fig. I). The thermal degradation of polyamide under specimens were placed in capped graphite crucible con- argon occurs close to 460C for a straight range of taining a powder bed made of a coarse alpha Sic temperature. The complete decomposition is achieved in owder (110 um)(Sika Nor Iv F120, Norton As only one stage. Whereas the polyamide thermal degra Norway) and 5 wt% of alumina(Ls3, Alcan Chemical dation under oxygen is divided into three stages dis to promote a protective atmosphere [13] tributed on a wide temperature range (300-550C) Dimensions and weights of green and sintered lami nar monolithic materials have been measured to deter mine green and sintered densities, linear and volume Te-CS under O2 shrinkage. Porosity(size, repartition, volume)has beer characterised by mercury porosimetry(AutoPore IV 三 PA-C under Ar 9500, Micromeritics-USA). The surface tension of 心 PA-C under C mercury and the contact angle between mercury and porous LPS-SiC have been, respectively, taken as 2 60 0.485 J/m and 130. Bulk density (ceramic mate- rial+ open porosity closed porosity) and apparent (only ceramic material closed porosity) of sintered samples have been also measured orosity has been characterised by image analysis performed on polished samples via an optical micro- cope. Nearly 1500 pores were taken into account. The ollowing parameters have been measured: Feret'di ameter, equivalent diameter, aspect ratio and surface 300400500 area Temperature (C) Grain size has also racterised by image Fig. 1. Variation of the PFA mass during thermal degradation(20C/ analysis performed on min) under O, or Ar gases(25 ml/min).       $! . 
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