Thermal stabilization of 2D carbon-carbon composite object geometry (2)

Study procedure: All measurements and testing during a study of the properties of carbon material and carbon-carbon composite material, and also monitoring production operations from CM preparation, their carbonization, compaction, and HTT, were carried out by means of metrologically tested measurement provisions. The relative change in specimens and object dimensions in three mutually perpendicular directions was determined by measurement with standard instruments.

Measurement of the linear thermal expansion coefficient (LTEC), and also values of carbon matrix shrinkage, were performed by direct determination of specimen elongation. Direct measurement of movement was carried out with a horizontal microscope. From the curve for the dependence of relative elongation on temperature measurements were calculated for the average LTEC within prescribed temperature ranges. The test temperature was measured with an optical pyrometer. Measurements were carried out in a laboratory electric vacuum furnace in a pure argon atmosphere. In order to reduce errors of optical measurement for temperature due to incomplete radiation of a gray body, and due to radiation of faces, a special form of specimen edge was developed. The absolute error of the method was ~0.1 × 10-6/K.

A study of dimensional changes during primary heating of carbon-reinforced plastic or carbonized specimen materials was carried out by a dilatometric method. Temperature was monitored by means of a chromel-alumel thermocouple. The error of determination did not exceed 10 K.

Physicomechanical properties of filaments of carbon threads at elevated temperature were studied using a specially developed high-temperature device in the temperature range from 293 to 2273 K. carbon filaments of round section ~7 μm in diameter were loaded by means of graphite grips in a vacuum chamber. The measurement error for strength was 15%, and for Young’s modulus it was 30%. The maximum absolute error in the range up to 3372 K was ±49K.

Temperature measurement in perfoirming HTT production operations in electric vacuum furnaces of different working volume was carried out with a Promin’ pyrometer, which during measurement of the production treatment temperature used the APIRS system as a standard. As a result of an increase in measurement error due to individual sensitivity of an operator, absorption by the atmosphere, and window sight glass, in accuracy of considering the degree of body blackness, and some other less significant factors, deviation from the true temperature is within the limits from -40 to +10 K of the nominal value.

Experimental results:

The high-temperature treatment currently adopted means the range of temperature at which there is complete removal from carbonizing matrix of the main volume of non-carbon atoms, up to temperature for completion of formation of a two-dimensional carbon crystal structure, within which there is no initiation of volumetric graphite crystallization. This temperature range is normally in a region from 1173 to ~2743 K. Above 2473 K there is graphitization, which completes formation of structural graphites, the main content of which is 3D crystallization.

One of the processes difficult to control, accompanying high-temperature processing, is distortion of billet geometric shape as a result of shrinkage, which may be irreparable due to existence of a change in the carbon matrix stiffness. The extreme form of distortion is warping and breakdown as a result of development of internal production stresses. This phenomenon has been studied previously and it has been revealed that with unfavorable conditions development of breakdown also occurs at the level of structural fragments of a composite.

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