High-temperature treatment of carbon-carbon composite materials (2)

Studies for measuring specimen dimensions during primary heating of carbon-reinforced plastics or carbonized specimens were carried out by a dilatometric method. Temperature was monitored by means of a chromel-alumel thermocouple. Determination error did not exceed 10 K.

The temperature for performing HTT in electric vacuum furnaces with a different working volume was measured by Promin’ optical pyrometer, which during measurements of the processing treatment temperature was used as a standard for the APIRS system. Temperature measured with a Promin’ instrument, according to test data, is lower by 10-20°C than the true values, caused by absorption of the atmosphere and sight window. Additional errors, caused by equipment, radiation devices, taking account of the different actural degree of blackness of treatment blank due to shrinkage of a charge, may be in total up to 14°C. In turn, the error for the radiation capacity and introduction of corrections for the sight glass compensate each other. As a result of an increase in measurement error due to individual operator sensitivity, absorption of the atmosphere, and glass in the measuring window, incorrect consideration of the degree of body blackness, and some othe less significant factors, deviation from the true temperature value is within the limits from -40 to +10°C of the nominal value.

Specific electrical resistance was determined by a potentiometric method by direct current using a standard. The drop in voltage at potential contacts of the measuring instrument was from 10 to 20 mV.Specimen dimensions were measured with an accuracy of 0.01 mm. On average the level of specimen electrical resistance was ~40 μΩ*m, and the error was ~0.36 μΩ*m.

The average value of thermal conductivity was measured in a steady-state axial heat flux according to procedure MI 00200851.125-2007, based on comparing thermal conductivity of test materials. In order to avoid convection the measuring cell was placed in a thermostat. The maximum calculated experimental error, taking account of measuring instruments and the different heat-resistance of a standard and a test specimens, did not exceed 7%.

Experimental results:

High-temperature treatment concerns the temperature range for complete removal from a composite of carbonizing matrix of the main volume of non-carbon atoms up to completion of formation of a two-dimensional carbon crystal structure, within which however volumetric carbon crystallization is not initiated. Normally this is the temperature range from 1173 to ~2473 K. Above ~2473 K there is graphitization, completing preparation of structural graphite, and mainly there is three dimensional crystallization.

Carbonization processes, high-temperature treatment, and then graphitization proceed under conditions of transient heating of the workspace of a furnace, or transient heating of a charge of product being treated. Therefore large blanks, whose size is commensurate with the dimensions of a furnace workspace, in individual areas simultaneously some individual parts at a certain time are in a different stage of physicochemical and structural transformations. Alternate regeneration of destruction of structural forms of a substance at a given temperature, recombination of newly forming structures, are accompanied by a change in physicomechanical properties, and relaxation of shrinkage processes of polymorphic transformations. As a result of this a billet loses its prescribed geometric shape. For production from composite materials, whose restoration of geometric shape is impossible by machining, a key question of the technology becomes establishing the temperature and time limits for structural transformation in order to regenerate the subsequent behavior of a process after total completion of preceding structure formation.

HTT was carried out under industrial conditions in electric furnaces of the SSHG type. Conical blanks with a greatest diameter of 2250 and height of 100 mm were placed at six levels over the height of a furnace workspace. The overall height of the heaters from the hearth of the graphite stand to the upper marks of the graphite crosspieces, connecting heaters in pairs, is 3970 mm. Heat fluxes from the heaters and from the inner surface of a graphite screen, increasing from the current lead of heaters to the graphite screen, act on blanks being treated. The overall height of a charge to the upper section blanks being treated is 3870mm. The height of the stand is ~600mm. The height of each level of blank arrangement is ~550mm. As a result of this not more than half of the height of the blanks is under direct radiation from graphite heaters. The rest of the height of blanks receives a heat flux from the inner surface of the outer covering blanks. Temperature monitoring by an optical method was carried through three sight glasses over the height and through two sight glasses located in one side at one height. Control of the process was carried out through one of the central sight glasses over the height.

 

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