Influence of temperature on phase, microstructure and oxidation resistance of carbon/carbon composites modified by hydrothermal treatmentby Huang

Surface Engineering

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Influence of temperature on phase, microstructure and oxidation resistance of carbon/carbon composites modified by hydrothermal treatment

J.-F. Huang*, N.-N. Wang, L.-Y. Cao and J.-P. Wu

Carbon/carbon (C/C) composites were modified by a hydrothermal treatment using phosphoric acid solution, B4C, SiC and Al2O3 powders as infiltration fillers. The influence of the hydrothermal treatment temperature on the phase, microstructure and anti-oxidation property of the as modified composites was investigated. The phase composition and microstructure of the modified composites before and after oxidation were characterised by X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy techniques. Results show that the oxidation resistance of the C/C composites is effectively improved after modified by the hydrothermal treatment. The mass loss of the C/C composites modified at different hydrothermal temperatures increases parabolicly with increased oxidation time, while the relationship between hydrothermal temperature and mass loss of the modified C/C composites after oxidation at 700uC for 10 h reveals a linear dependence. The mass loss of the modified C/C composites is only 3?6961023 g cm22, which is much lower than 141?5561023 g cm22 of the non-modified C/C composites, after oxidised at 700uC in air for 10 h.

Keywords: Carbon/carbon composites, Oxidation, Hydrothermal, Matrix modification, Morphology, Surface structure

Introduction

High performance carbon/carbon (C/C) composites possess many merits such as light weight, high specific strength, good friction and wear properties, high toughness and thermal stability, which make them irreplaceable for applications in aeronautics industries and aerospace places. Oxidation of the C/C composites under oxygen containing conditions .400uC will result in the degradation of their strength which is extremely crucial for the applications at high temperatures.1–3 Recently, multilayer and functionally gradient ceramic coatings have been intensively developed by many methods including pack cementation,4–6 in situ formation process,7 plasma spray,8,9 sol–gel,10 chemical vapour deposition,11 etc.

Although ceramic coatings could protect C/C composites from oxidation at high temperature, it could not efficiently protect C/C matrix at low temperature, especially at 400–900uC for the existence of microcracks that generated in the coatings due to the mismatch of thermal expansion coefficient between the carbon matrix and ceramic coatings, and the microcracks could not be self-healed by the coatings at that temperature range.

Therefore, it is important to improve the oxidation resistance of C/C matrix at low temperature. Matrix modification process is a reasonable route to achieve this purpose. Addition of some oxidation inhibitors into the bulk C/C composites has been found to be an effective method to improve the anti-oxidation property of C/C matrix. The fabrication of ZrB2 mixed C/C composites has been studied.12 It was found that the oxidation resistance of the as received ZrB2 mixed C/C composites was obviously improved due to the formation of B2O3 on the composites surface and intermatrix defects. An alternative method which gives reliable oxidation protection to C/C composites involves infiltration of inhibitors into the interbundle pores of the C/C composites by a slurry infiltration process13 or immersion infiltration process.14 In the present work, in order to effectively improve the oxidation resistance of C/C matrix in an easy way, a novel hydrothermal technique was adopted to modify the C/C matrix. The influence of the hydrothermal treatment temperature on the phase, microstructure and oxidation resistance of the as modified C/C composites was particularly investigated.

Experimental

Small specimens (10610610 mm) used as matrix were cut from bulk 2D-C/C composites (prepared in Xi’an,

China) with a density of 1?70 g cm23. Before the

Key Laboratory of Auxiliary Chemistry & Technology for Light Chemical

Industry,Ministry of Education, Shaanxi University of Science &Technology,

Xi’an Shaanxi, 710021, China *Corresponding author, email huangjf@sust.edu.cn  2011 Institute of Materials, Minerals and Mining

Published by Maney on behalf of the Institute

Received 20 October 2008; accepted 31 December 2008 320 Surface Engineering 2011 VOL 27 NO 5 DOI 10.1179/174329409X409378 hydrothermal treatment procedure, the specimens were hand polished using 240, 500, 800, 1200 and 1500 grits

SiC papers with a later ultrasonic cleaning using ethanol and distilled water; and then the C/C specimens were dried at 110uC for 2 h. Suspension solution compositions for the hydrothermal treatment were as follows

B4C (800 mesh)/SiC (800 mesh)/Al2O3 (300 mesh)/ phosphoric acid5(1?3–1?6) g/(0?75–1?0) g/(0?25–0?4) g/ 28 mL

All the above raw materials were analytical grade.

First, the infiltration suspension mixtures were mixed and then put into a hydrothermal autoclave under agitation with the filling ratio of 70%. Second, the polished C/C specimens were added into the above autoclave and immersed in the suspension mixtures.

Next, the autoclave was placed into an oven and heated to a certain temperature, subsequently kept at that temperature for 60 h. After the hydrothermal treatment, the autoclave was cooled down to room temperature in air. The treated C/C specimens were picked out from the suspension mixtures and cleaned with ethanol. After that, the modified C/C specimens were dried at 300uC for 10 h.

The as modified C/C specimens were heated at 700uC in air in a muffle furnace to investigate their isothermal oxidation behaviours. Cumulative weight changes of the specimens were made at intervals and the procedure was repeated. The masses of the specimens before and after oxidation were measured using a precision photoelectric 1 Surface XRD patterns of modified composites treated at different hydrothermal temperatures a 120uC; b 150uC; c 180uC; d 200uC; e magnified image of b; f magnified image of c; g surface EDS analyses of e; h spot