1、 Microstructure and solid particle erosion of carbon based materials used for the protection of highly porous carbon-carbon composite thermal insulation R. I. BAXTER, R. D. RAWLINGS Department of Materials, Imperial College of Science, Technology and Medicine, London SW7 2BP, UK Multiparticle erosio
2、n tests were performed on candidate coating (colloidal graphite paints) and cladding (dense carbonc arbon composites and graphite foil) materials employed to protect porous carbon carbon composite thermal insulation in vacuum and inert-gas furnaces that utilize inert gas quenching. The dependence of
3、 the erosion rate on the angle of incidence of the erodent was examined and related to the microstructure and the mechanisms of material removal as observed by SEM. In addition, the effect of a thin chemical vapour deposited (CVD) carbon layer on top of a colloidal graphite paint coating and a graph
4、ite foil clad was investigated. The coating and cladding materials displayed a greater erosion resistance at all angles of incidence compared to the porous carbon carbon composite. In general, the greatest erosion rate was found at an angle of incidence of 90, where the erodent stream is perpendicul
5、ar to the erosion surface, and brittle fracture was the predominant mechanism of material removal. The exception was the graphite foil material which displayed maximum erosion at an angle of incidence of 60. For this material, two mechanisms were effective: disruption of the graphite akes, which are
6、 mainly held together by mechanical locking, and a ploughing-like mechanism. The addition of a thin CVD carbon layer to colloidal graphite paint improved performance, whereas the erosion resistance of the graphite foil was slightly degraded as the CVD layer was too thin to prevent the ploughing-like
7、 mechanism. 1. Introduction A class of highly porous carbon carbon (C C) composites, with low densities in the range 0.1 0.4 Mg m3, are utilized as thermal insulation in vacuum and inert-gas furnaces at temperatures up to2800 C. A consequence of the vacuum-moulding process used in the production of
8、the composite is that the discontinuous fibres are orientated into layers to form a two-dimensional planar random structure. The vast majority of the volume of the composite consists of interconnected pores and the fibre network is bonded at the intersections of fibres by discrete regions of the car
9、bon matrix as opposed to a continuous matrix. For this reason these composites are also known as carbon bonded carbon fibre (CBCF). As a result of the high porosity and the fibre orientation,the thermal conductivity perpendicular to the fibre layer planes is low, a typical value for a material with
10、a nominal density of 0.20 Mg m3 is 0.24 W m1 K1 at 2000 C in vacuum . Investigations into the microstructure 3, 4, mechanical properties 2, 5 9 and thermal properties 10, 11 of these materials have been reported. ( 1997 Chapman & Hall CBCF is used in furnaces employed in high technology applications
11、 such as single-crystal growing (for example, silicon or gallium arsenide) or metal heat treatment. The heat treatment of metals, such as tool steels, is increasingly carried out in furnaces that utilize gas quenching (typically nitrogen is used) 12, 13. The gas quench may be used to reduce the turn
12、around time of batch processes or as an integral part of the heat-treatment regime. The advantage of gas quenching during heat treatment, as opposed to an oil quench, is that the cooling rate can be controlled； therefore, it is possible to reduce warping and cracking in the component . During gas qu
13、enching, parti- culate matter may become entrained in the gas ows, and impingement with the insulation may result in material removal. In the challenging environment of gas quenching, there is a requirement for erosion protection of the CBCF by the use of higher density carbon-based coating and clad
14、ding materials.Generally, ductile and brittle materials exhibit different erosion characteristics； of particular interest is their relationship between the erosion rate and the angle of incidence 15. Ductile materials tend to display maximum erosion at glancing angles of impact, approximately 30 for
15、 metals, and material removal is thought to occur by a micromachining mechanism with a contribution of deformation wear at higher angles. On the other hand, for brittle materials, maximum erosion is found where the erodent stream is perpendicular to the erosion surface, and material removal typicall
16、y results from the formation of Hertzian or lateral cracks . Although it is a convenient approach to idealize materials erosion behaviour in this manner, it is an oversimplification, because erosion is found to depend on other factors, including the erosion conditions, such as erodent par- ticle siz
17、e and shape, as well as the details of the microstructure of the target material . This paper is concerned with the examination of the microstructure and the efectiveness in improving the erosion resistance of several candidate coatings and claddings. The results presented involve the steady state e
18、rosion rate as a function of impingement angle under defined conditions. The overall aim of this work is to relate the microstructure to the erosion data by means of a mechanistic approach .materials included the Fiber Materials Inc. C3 composite, which is resin impregnated, and the Toyo Tanso G3470
19、 . In addition, a high-density carbon carbon composite was produced by employing CVD over a period of 800 h to infiltrate a 5 mm thick section of the CBCF substrate to a density of 1 Mg m3. The CVD process used natural gas as the carbon precursor and nitrogen as the carrier gas. The densification wa
20、s carried out at approximately 1100 C under a reduced. 2. Experimental procedure 2.1. Materials The CBCF used as the substrate was a standard commercial material (density 0.18 Mg m3) manufactured by Calcarb Ltd. The coating and cladding materials were applied to the xy plane of the CBCF substrate (s
21、ee the schematic diagram of CBCF structure in Fig. 1)； the xy plane is perpendicular to the direction of minimum thermal conductivity and hence is most likely to be the exposed surface of the insulation in a furnace. The coating and cladding materials exam- ined in this paper were all carbon based a
22、nd they are listed in Table I. The coating materials are defined as those that bond independently to the CBCF substrate,whereas the claddings are bonded by means of a car- bonizing cement. Calcoat and Calcoat M are colloidal graphite paint coatings that were applied to the CBCF substrate by brushing
23、. The material was subsequently heat treated at 900 C in nitrogen to carbonize the resin constituent of the colloid. Higher density carbon carbon composites (1.3 Mg m3) used as cladding pressure of 5 kPa. (Note that the CVD of carbon in the interior of a porous medium is sometimes termed chemical va
24、pour infiltration, CVI.) Another cladding material was graphite foil which was produced by Toyo Tanso by compressing exfoliated graphite akes in a rolling operation 23. The foil is exible in nature and is predominantly held together by mechanical locking, as no binder is used. Further samples were p
25、roduced by subjecting the Calcoat coating and the graphite foil to a CVD treatment (samples desig-nated#CVD in Table I) for a period of 75 h under the conditions described above. A more extensive descrip- tion of the materials will be forthcoming in the dis- cussion on the microstructures. 2.2. Eros
26、ion testing Multiparticle erosion tests were performed on a gas- blast type rig, as described by Carter et al. 24. In this apparatus the erodent particles enter the rig via an aperture in the base of an open hopper. A venturi fitted in the system allows the particles to be entrained in the compresse
27、d air ow. After passing through a nozzle with an 8 mm internal diameter, the particles strike the target at a stand-of distance of 40 mm. The target specimens had nominal dimensions 25 mm； 12.5 mm；5 mm. The erodent used was angular equiaxed silica sand obtained from Hepworth Minerals and Chemicals L
28、td, Redhill, UK. The erodent was sieved to particle sizes between 150 and 300 lm, the mean size (by weight) was 230 lm which was found by a laser difrac- tion method (Mastersizer 1005, Malvern Instruments Ltd, Malvern, UK). The velocity of the particles was 6 m s1, found by the streaking camera technique at the
