18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS Effect of deposition position on microstructure of HfC coating fabricated by low pressure chemical vapor deposition Y ． Wang 1 , H. Li 1 *, Q Fu 1 , H Li 1 1 C/C Composites Research Center, Northwestern Polytechnical University, Xi’an, China * Corresponding author ( lihejun@nwpu.edu.cn ) Keywords : Deposition position; HfC coating; microstructures; chemical vapor deposition In this paper, HfC coating was prepared on 1. Introduction the surface of the C/C composites, and the effect As ablation resistance material, C/C composites of deposition position on the microstructure of have unmatched properties compared with the others, the HfC coating was studied. such as the low density, low CTE(coefficient of thermal expansion) and their higher thermal strength 2. Experimental even above 2000 ℃ ; So the composites have 2.1 Experiment materials become the ideal option for the nose cone and leading edge at supersonic aerocraft [1,2] . However, Small specimens (50mm×10mm×3mm) used as the C/C composites are very apt to be oxidized at the substrates were cut from bulk 2-D C/C high temperature. Aiming to this, many Si-based composites with a density of 1.71g/cm 3 . These ceramic coating systems were developed and can specimens were hand-polished using 400 grit SiC protect the composites for long time under 1700 ℃ [3] . paper, then cleaned with distilled water and dried at But, for the oxidation and ablation resistance at 393K for 2 h. higher temperature, especially under the particles flow with high speed, these coating become disabled. HfCl 4 (Alfa Aesar, 99.9%) and C 3 H 6 (>99.9%) So, the carbides with higher melting point should be were used as the precursor of Hf and C. The Ar and introduced into the coating in order to improve the H 2 used in the deposition are both high purity resistance ability at high temperature [4] , such as HfC, (>99.999%). TaC [5] , NbC, ZrC [6,7] et al; and among these, HfC has 2.2 Preparation of HfC coating the highest melting point of 3890 ℃ , and relative better thermal stability, therefore, it becomes the The as-prepared C/C samples were hanged ideal coating material for the ablation resistance for with carbon fiber in the reaction chamber supersonic fly. equidistantly, from 15cm to 30 cm away from the gas inlet. And the samples were labeled a , b , c , d Compared with the other methods (sol-gel [8] , respectively, as seen in the Fig. 1. spraying [9] ), plasma HfC coating obtained through chemical vapor deposition has more dense The deposition was carried out in a two- structure and better design ability, so the CVD temperature zone CVD furnace. The HfCl 4 vapor method is always firstly adopted when a multilayer was produced by heating HfCl 4 powder at lower system is needed. However, the HfC microstructure temperature zone of the furnace, with H 2 being the is sensitive with the deposition craft, and HfCl 4 , the carrier gas. All the gases were mixed in the gas solid precursor of Hf, is hard to be well controlled. mixer at before injected into high temperature So, to get a coating with a better ablation resistance, reaction chamber. The reaction of HfC deposition the rules between microstructure and deposition process can be prescribed by the formula 1, as craft should be well studied. Up to now, rare reports follow: can be found about the HfC coating prepared by (1) 1/ 3 4 HfCl H C H HfC HCl + + ⎯⎯ → + CVD. 4 2 3 6 1
Fig.3 shows the texture coefficient of HfC 2.3 Test methods coating at different position, it can be found that The grown morphology was observed through with the position lifting, the TC of planes changes a Field emission scanning electron microscope lot. In the upstream of gas flow, (111) plane has the (Supera-55); the phase constitution was examined by highest TC, the TC of (200) is the lowest. With the X-ray diffraction (Rigaku-D/max-3C). changes of position, TC of (111) plane decreases According to the results of XRD patterns, the gradually, but the TC of (200) increases adversely. texture coefficients of different crystal plane were Slightly changes happened to the TC of (220) and also calculated through Harris’s method. Meanwhile, (311) planes. It is can be illustrated that growth the grown orientation at different position was orientation of HfC coating transferred from (111) to analyzed by the changes of the TC; the formula used (200), which can also be exhibited in the XRD was presented as follow [10, 7] : patterns (Fig.2): the intensity’s increase of the (111) and (222) peak, the decrease of the (200) and (400) / I I peak intensity. 0 i (2) TC = n × ∑ (1/ ) ( / ) When the gas flow passed from inlet to outlet, n I I 0 i the reactants were consumed gradually. So there will 1 i = be great concentration diversity generated in parallel TC: the texture coefficient of one plane; direction of the flow. In the upstream, the I i : the intensity of crystal plane as detected; supersaturation of reactants’ concentration was I 0 : the intensity of crystal plane in the standard higher; the grown process exhibited high nucleation XRD pattern from ASTM (American rate, no obvious preferred grown orientation Society for Testing Materials Index); appeared; while in the downstream, the n : numbers of the crystal planes. supersaturation dropped tempestuously, and so did the nucleation rate. Therefore, the coating became to 3. Results and discussion grow toward a preferred orientation: (220) plane. 3.1 Phase composition of HfC coating 3.2 Microstructure of HfC coating From Fig 2, it can be seen that the HfC coating The changes on the reactant concentration has single phase composition at position a . No peaks resulted in not only the grown orientation of C phase were detected. With the position lifting, transformation, but also the morphology of the weak peaks of C appeared in the HfC coating at coating, as shown in Fig. 4. In position a , the surface position b . At position c , the C peak intensity is compact and flat, the grains are spherical with a continued to increase; that is, it can be included that large size. With the position lifting, the size of HfC the C content increased with the position lifting. grains decreased. The space among the grains While at d , which is in the downstream of gas flow, enlarged obviously, the coating surface became to be the main peak of C became the strongest one, it is rough. In the downstream, the grain size diminished because that the C intensity mainly came from the continuously and presented “shadow effect” because matrix; in the downstream of gas flow, the of the so thin film caused by the low deposition rate. deposition rate was too low to form a thick layer. The coating was too thin for the X-ray diffraction When the reactants entered into the high- test, and the C of matrix can be easily detected. temperature zone, the deposition reaction started immediately; and with the consumption of HfCl 4 , Meanwhile, it is remarkable that the peaks of C 3 H 6 , H 2 , the reactant concentration began to reduce. HfC appeared a certain shift to the left at position a . In the upstream, the concentration was higher than It is because that the Hf was excess relatively, HfC the other regions and behaved a higher nucleation coating was apt to exist as carbon deficiency form: rate, which was propitious for the grown up of the HfC 0.6-0.99 , which is coincided with the normal grains and the inosculation between them. Therefore, existence of HfC in the form of metal excess [11] . the grains looked big and little space among the 2
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