ELECTRONIC LIBRARY
Karakuts Irene Sergeevna
LUDMILA KALAFATOVA
An estimation technique of defectness of the processed pyroceramics surface, based on the luminescent fault detection has been worked out. This method is combined with chemical etching of every layer. The calculation of the defectness parameters has been carried out on the basis of the automatized system of microphoto analysis of the defective surface.
Key words: cetalls (pyrolytic ceramics), presence of defects, grinding, technique of the definition of a defect stratum, fluoroscopic flaw detection.
1. INTRODUCTION
The development of modern mechanical engineering is connected with the development and introduction of new materials and progressive processes of their handling. Cetalls - a variety of glass ceramics (the other name is pyrocers or pyrolytic ceramics) can be, with good reason, referred to such materials. Having the increased fragility and low strength of extension and bending strength, they have at the same time a number of valuable properties: heat resistance up to 9000C, heat resistance under a load up to 12000C, compression strength 1600 MPa, microhardness HV= 8500 MPa. Cetalls are not subjected to corrosion, agressive media and are radiotransparent. The enumerated properties determine the area of their application: radioelectronics engineering and chemical industry, nuclear power engineering, aircraft and rocket industry, space engineering.
It is experimentally established, that the operating characteristics of products made of cetalls, and first of all durability and heat resistance essentially depend on depth and structure of a surface defect layer caused by machining. That is why the first requirement put in manufacturing of responsible products made of glass ceramics is to reduce the presence of defects on a treated surface to minimum. For its observance it is necessary to have a reliable technique of the definition of presence, depth and structure of the defect stratum in glass materials. The offered paper deals with this problem.
2. TECHNOLOGY OF GETTING PRODUCTS MADE OF CETALLS
2.1. Cetalls' properties and structure of their defects presence
Cetalls represent inorganic materials obtained with the help of thin crystallization of glasser or melts of various structures. The mechanical properties of cetalls are determined on the whole by their structure - size of crystals, amount of crystalline and glassy phases, their mutual distribution. The crystallization peculiarities ensure a high homogeneity of their microcrystalline structure. This is the reason of high strength properties and low machinability of the cetalls. In the present paper the cetalls (for example the cetall AC-418) are considered to be constructional materials used for manufacturing of such products as thin-walled envelopes of the complicated space forms and maintained in a wide range of temperatures with various combinations of mechanical and thermal loads.
In modern applied material science the defining role of defects presence in forming of strength properties of fragile constructional materials is conventional. The imperfections of the cetalls structure can conditionally be divided into the following categories (Table 1) [2].
It is microscopics defects presence that mainly defines really achievable levels of the cetalls strength if to compare it with all the other types of defects presence. The method of combined hardening [2, 3] is one of the rather widely-spread methods of reduction of defects presence of the treated surface of made from the cetalls products, and as a result, of raise of their constructional durability and op¬eration reliability.
It includes a diminution of the level of surface defects presence by etching in acids and consequent blocking of surface imperfections with contracting ionic ex¬change stress-hardening by ionic interchanging. The latter is implemented with immersing of products or the samples into liquid melt of special salts, which chemical composition is selected in such a manner that in it thereaction of substitution of ions with a smaller radius, for example, Li+1 - from cetall on ions with a large radius Na+1 - from the melt takes place. So in strengthened state the cetall AC-418 exceeds the non-strengthened its version on durability in 1,6 and on thermal resistance in 1,8 times.
Table1
Aspects of the defects presence of the cetalls
|
Category of imperfections
|
The
reason of origin
|
Aspect
and approximate sizes
|
Methods
of a research
|
|
Macroscopic
|
Deviations
from the technologyof deriving of the cetall
|
Bubbles
or bowls with a size of 0,6-4,0
mm
|
Visually or by optical way
|
|
Submicroscopic
|
Interior microstresses with crystalli-zation
|
Heterogeneities of thestructure with the size nomore than 1mm
|
Electronic
microscopy
|
|
Microscopic
|
1. Initial defects
presence of the structure - function
of the composition,
structure and mode of deriving of the cetall
|
Microcracks in residualglassphase and on theboundaries of the unit ofphases with the size from 6 up to40 mm, located in volume of
the cetall evenly
|
X-ray, electron and
structural analysis. exzoelectron emission etc.
without or in thecombination with the level-by-level deleting of the defect stratum by thin
grinding or polishing;
Offered method of
luminescene defects presence
|
|
2. Delects presence caused by machining
|
Microcracks of the various form located on a surface and adjoint to its surface, depth up to 300-400 mm
|
2.2. Technology of machining of products made of the cetalls
As was already told earlier, the cetalls refer to a category of difficult machining materials. Large machining allowance (10 mm and more) because of impossibility of deriving of exact workpieces are a peculiarity of large-sized products machining, the products having a complicated cetalls space form. In these conditions the technological processes (TP) of machining differs by many-staged character, that is illustrated by the Table 2.
Table2
Stages of machining of products of the cetalls
|
Operations of machining
and the applied tool on the stages
|
Operations
of combined hardening:
|
|
preliminary forming
|
final
forming
|
finishing
machining
|
deep etching
|
ionic
hardening
|
|
Coarse
grinding by the diamond tool
|
Fine grinding by the diamond tool
|
High finish or lapping
with diamond grind-bars or pastes
|
Deleting of the layer by depth up to 0,8 mm in a solution of
acids
|
Handling in melt of
salts from 1 till 3 hours
|
The complexity of each stage of machining approximately corresponds to the ratio 1:3:12. The common duration of machining of large-sized products can reach 90 and more hours. Thus, the defect stratum is formed which, with a process engineering existing on production, can reach 0,5 mm and more. All this layer should be deleted at the operation of hardening of details completing the process of manufacturing. Taking into account complexity, high cost and ecological danger of the operation of hardening, the provision of minimum on depth and favorable structure of defects presence of the treated surface in a combination with sufficient high level of efficiency of the process of grinding determines efficiency of the process of machining. The solution of this task is impossible without development of reliable methods of diagnostics of qualitative parameters of the formed surface.
3. DEVELOPMENT OF THE TECHNIQUE OF EVALUATION OF DEFECTS PRESENCE OF THE FORMED SURFACE OF PRODUCTS FROM CETALLS
The parameters of the formed defect stratum are influenced by conditions of machining and, first of all, magnitude of forces, influencing the treated surface of the detail [1]. With grinding the force characteristics of the process are determined by a series of the factors: by durability of a treated material; by the scheme of grinding; regime parameters of the process; by technological parameters of the tool, by cutting ability depending on a degree of a wear of abrasive grains; by physical-and-chemical properties of the used technological medium, its relation-ship with the treated material. With a comparison of variants TP, which differ by one or several enumerated factors, depth and structure of the defect stratum formed as a result of machining can serve as a criterion of their efficiency.
Nowadays there is a significant amount of methods of diagnostics of a condition of the treated surface, depth and structure of the layer, broken by machining, (see Table 1). A certain their part is founded on the usage of not destroying methods of monitoring - X-ray, various aspects exzoelectronic emission, electronic - microscopic, electronic and graphic and so on. Their shortages: the possibility of detection of parameters of a broken stratum with depth no more than 5 micrometers; getting of rather authentic outcomes with machining only homogeneous by structure materials with stable physical-and-mechanical performances subjected to thin grinding and polishing for example various aspect of crystals used in electronic industry; impossibility to estimate qualitatively and quantitatively a modification the structure and sizes of imperfections at various levels of lying on the treated surface; expensiveness and complexity of a realisation under production conditions. The indicated restrictions can partially be removed with use of the mentioned above methods in a combination with level-by-level deleting of a broken layer of the material. The delete can be carried out in a physical way (thin grinding or polishing), by means of chemical etching or with a combination of both modes.
The analysis of effectiveness of application of above mentioned methods of diagnostics in conditions of machining of large-sized products of glass-and-crystal materials distinguished presence of many stages, significant allowances on machining essential by force influence on the treated material, has shown the following. The technique of the research of the broken layer of a material should fulfill a series of functions: to allow to determine the structure of the defects presence sequentially on various depth, not bringing in distortions at the expense of additional, first of all force, influences on the material being the basis of the technique (therefore the use of grinding or polishing of a material with level-by-level depth is inexpedient); to ensure with sufficient reliability the definition of depth of the broken stratum, filing and monitoring of the parameters of the defects presence; using program methods, to estimate automatically the level of defects presence, stimulated by that or other technological influences.
The capillary method of fluoroscopic flaw detection in a combination with level-by-level chemical etching corresponds to the conditions formulated above [1, 2]. The capillary method of the definition of the defects presence is based on a capillary penetration and contrast of the flaw-detection materials (luminescene of liquids) in optical radiation. The effect imperfections registration is made more vivid with the help of the tools promoting the most full manifestation of the indicator substance. In the case under consideration luminescent liquid exhibited in a ultra-violet radiation was used.
The Figure 1 illustrates the general scheme of the tested samples handling cycle when the defects presence of the broken materials layer is identified by the given method. The number of operation cycles of the samples is determined by an amount of levels of etchings, on which it is necessary to investigate a defect structure of the cetall.
For chemical etching of the samples the solution of hydrofluoric and sulphuric acids of the following concentration was used 20% HF and 1% H2S04.
The level of sensitivity of the method depends on properties of the indicator liquid. During the researches the indicator luminescent liquid LL-6A was used ensuring the first (highest) level of sensitivity of the method. The sensitivity of the method was determined on the bases of quantitative evaluation appearance of the standard imperfection in view of geometric and optical singularities of its indicator (luminezing) track. The standard imperfection was plotted on a sample with the help of diamond Vickers' pyramid with its impression and scratching of the polished sample of cetall. The true sizes of an imperfection were determined on the microscope with 1500 multiple magnification. It is established, that the method allows to reveal imperfections by an expansion from 0,1 mm up to 1 micrometer. The deviation of an indicator track from a true size of an imperfection docs not exceed +10%.
The control of the structure of the defects presence of the cetall layer, broken by grinding, on various depth was carried out by microphotos of the most characteristic sites of the surface of the sample. The precision photoshooting of the surface was carried out on luminescent microscope, permitting to observe the object of the research in ultra-violet radiation.
The magnitude of the layer of the material deleted with etching, was measured from the single basis with installation of the sample on desktop of the microscope by a turn of its micrometric screw. The measurement accuracy is 1 micrometer. This method allows to receive the undistorted picture of the defects presence of the broken layer at various levels, beginning from the level of the treated surface and finishing with the exit on the depth characterized by a lack of tracks of destruction of a material, caused by machining.
The microphotos of the defects presence of the cetall AC-418 at various levels of placement, obtained with use of the method of luminescent flaw detection are given.
The control of the structure defects presence on various depth was carried out on microphotos of the most typical sites of the surface of the samples. Depth of penetration of the imperfections (h), relative magnitude (square) of the size of single maximum microimperfection (Srel), which is at the consider site of the treated surface and the level of etching: Sr = n*Sin, where Sin - square of the initial imperfection of a structure of the cetall, accepted for a unit, magnitude known for each brand of the cetall, n - factor of the coordination of sizes of imperfections-structural and brought in by machining; a relative denseness of imperfections Sc. defined by the ratio of square occupied by imperfections, to all researched square of a sample arc chosen to be the criterion of evaluation of the structure of the defect layer.
Earlier calculation of the parameters of the defects presence was carried out manually with the help of cross-section grids superimposed in the microphoto The whole procedure of the calculations supposing with a variation of only one of entering technological parameters machining at not less than a hundred and fifty photos, manually covers a significant amount of time, differs by subjectivity of estimations and requires a significant number of evaluations connected with the definition of both sizes of imperfections and statistical handling of obtained outcomes.
The enumerated difficulties constrained the application of the method of capillary flaw detection under production conditions. To overcome them the automated system of the analysis of microphotos of the defect surface of the samples was developed [4]. The system is constructed on the basis of a complex hardware (PC IBM PC/AT and the scaner) and special software. With the help of the scaner the microphotos are given figure value and are introduced into memory of PC. The complex of the programs allows to raise automatically visibility of photos, to select the areas of imperfections and to calculate averaged and integrated performances of imperfections to state mathematical assotiations of the parameters o the defects presence from varied technical characteristics of the process of machining.
With the purpose of validation of the outcomes of automatic calculations o the photos the parameters of the defects presence were parallely determined manually with the help of cross-section grid. The error of measurements of each of considered parameters connected with the adjustment of initial photos and peculiarities of the automated score made up from 6,2% up to 12,4%, which is quite admissible for the engineering researches. The application of the automated system of the analysis of microphotos allows to increase velocity of calculation ii comparison with hand-operated variant in tens time with observance of a required accuracy of accounts of the parameters of the defects presence.
4. CONCLUSION
The considered technique allows to estimate a degree of influence of data-ins of the process of cutting on the depth and structure of the defects layer the scheme of grinding; technological and design features of the diamond tool, necessary level of its cutting ability; conditions of grinding; an aspect of a technological medium and so on. The obtained information is defining with assigning rational values of machining allowances for each passage of abrasive machining. Because of obtained outcomes the recommendations for organization of the technological process of machining of products from the cetalls are developed [1].
REFERENCES
[1] Burmistrov V., Kalatalova L., Raikhel A., Dependence of the defects presence of products of
the cetalls on technological parameters of the process of grinding, Ukrainian Institute of Glass,
Remembering Transactions, 1994, Issue l, pp. 171-179.
[2] Dubovick V., Raikhel A., The defects presence and levels of durability of the cetalls, Problems
of Durability, 1984, no. 7, pp 79-84.
[3] Dubovick V., Raikhel A., Ivchenko L., Longevity in conditions of curving of combined and
strengthered the spodumen cetall, Problems of Durability, 1990, no. 2, pp. 126-I28
[4] Kalafatova L, Diagnostics of qualitative performances of the grinded products of engineering celalls, in: High process engineering in a mechanical engineering: diagnostics of processes and security of quality. Materials of a 6-th international scientific and technical seminar, Kharkov, KSPU 1996, pp. 61-63.