CC BY
21
UDC 537.525.7:621.762
V. Khristoliubova, I. Abdullin, А. Khubatkhuzin
INCREASE OF HARDNESS AND PHYSICAL MECHANICAL PROPERTIES OF METALS AND TUNGSTEN COBALT ALLOYS WITH THE LOW PRESSURE RF-PLASMA
Keywords: RFplasma, alloy, low pressure.
A coating on the surface of solid alloy with the help of high-frequency discharge of low pressure due to its interaction with the workpiece was formed. As a result offorming of a coating on the surface of the metal increasing of hardness and physical and mechanical properties of metals was produced.
Keywords: ВЧплазма, сплав, пониженное давление.
Получено покрытие на поверхности твердого сплава с помощью высокочастотного разряда пониженного давления при ее взаимодействии с обрабатываемым изделием. В результате формирования покрытия на поверхности металлов получено улучшение физико-механических свойств металлов и повышение твердости.
The question of improvement of quality, reliability and durability of details of the equipment is one of the fundamental in the industry of mechanical engineering of Russia. Sealing devices very often have the form of rings for installation between motionless part and rotary part of the element. They are applied in the rotating equipment. Such kind of devices are intended to divide two environments for the purpose of prevention of leakage of the fluid environment and to provide tightening of the shafts which transfer mechanical energy to the working body of the mechanisms, such as pumps, compressors. Due to the influence of extreme conditions, for example corrosive medium, the unfavorable moments at start-up and turndown of compressors, they can easily collapse [1]. Besides, a sealants are exposed to considerable wear and can break in the equipment, especially because of a friction and therefore the replacement is often demanded. Sealants are made of the firm alloys executed on the basis of carbides of tungsten, the titan and tantalum by a method of powder metallurgy industry. As a sheaf cobalt is used. Now three groups of firm alloys are issued in Russia: the one-carbide -tungsten, two-carbide - titan-tungsten and three-carbide -titan-tantalum-tungsten.
Now in the domestic firm alloy industry the in-depth researches connected with expansion of scope of application and possibility of increase of operational properties of firm alloys are conducted. One of effective ways of increase in service life of products of mechanical engineering is modification of properties of the surfaces which are exposed to wear. Results of pilot studies of processes of wear and destruction of various products at their operation showed that reliability of a product and service life depends on, and quite often are completely defined by a condition of a blanket [2, 3].
Perspective method of processing of materials is influence of radio-frequency (RF) plasma of low pressure. The result of a method is an ionic implantation of atoms of plasma-forming gas in the volume of metal up to 100 nanometers in depth [4].
The advantages of influence of radio-frequency plasma of the low pressure are: almost unlimited resource of work; simple instrumentation; small duration of processing; possibility of combination of several technological operations; high density of the coverings, which equals to density of an initial material, etc. Use as
a working body of various plasma-forming gases allows to process details of a difficult configuration, including internal surfaces. There is a change of phase structure and material structure due to the impact of RF plasma of the low pressure on metals and alloys. It leads to improvement of several, sometimes opposite, properties simultaneously. For example, it is probable to increase microhardness and fatigue durability and longevity at the same time. Distinctive feature of offered technology is use of "cold" plasma: gas temperature in a plasma stream can be regulated in the range from 40 to 600C. Energy of ions is enough for curing of micropores and microcracks, elimination of fractured and relief layers, formation of the squeezing residual tension in a near-surface layer of a sample, etc. The result of influence of RF plasma of the low pressure is saturation of blankets by atoms of plasma-forming gases (Ar, N, O, S). Nanodiffsion covering on the detail surface is formed. It changes a chemical composition of a blanket, blanket structure, microgeometry of a surface (roughness), a power stock of a blanket.
As the surface of sealing rings is exposed to wear it is necessary to carry out material hardening by its gas-saturation by atoms of carbon and formation of carbides of the metals which is a part of a product. For the purpose of increase of mechanical properties of details of compressors samples of the sealing rings made of a firm alloy of tungsten- cobalt were processed. The sample entered into RF plasma discharge is exposed to bombing by ions of plasma-forming gas. This causes a recombination of ions, cleaning of a surface from various pollution, dispersion and a burn-off of microledges, curing of microcracks. That in turn leads to reduction of a roughness of a surface, to change of a structure and structure of a near-surface layer that provides wear resistance increase. Samples were placed in vacuum installation, the scheme is presented on fig 1.
Pilot studies showed that changes in a blanket of materials are connected with hardness and
reaction chamber, 4 - device of purification of argon, 5 - rotary device, 6 - processed detail, 7 - vacuum camera, 8 - pumping system, 9 - control management system, 10 - thermocouples, 11 - gas supply system
Pure argon was used as working gas during the research of processes of finishing cleaning and nanopolishing of surfaces. Mix of gases from argon propane-butane in the following modes was used for impact on structure of a surface of a product (Tablel).
Table 1
№ Gas Pressure, Pa Plate voltage, kV Plate current, А Flow, cm3/min
1 Ar 22 7 0,5 1500
Лг+(СзИ8+ С4Н10) 21 7 0,5 1300+200
2 Ar 26 7 0,5 2000
3 Ar 26 5 0,5 2000
Лг+(СзН8+ С4Н10) 26 5 0,35 1600+400
In all experiments was a negative potential on a product about -20 V for the purpose of increase of concentration of electric field near details.
Samples were placed perpendicular to a stream. Samples were degreased and dehydrated before plasma processing for elimination of side effects. The magnitude of a sample temperature at the establishment of regularities of change of properties of a blanket from plasma parameters was chosen, on the one hand, to intensify plasma processes, and on the onother hand -heat treatment wasn't a dominating factor at this temperature. It is experimentally established that time of achievement of working temperature and receiving of a uniform distribution of temperature on all volume of a material is 15 - 20 minutes. Therefore all products were processed in plasma of pure argon within 25 minutes, then 20 minutes in mix of argon with propane-butane.
Microhardness and roughness measurement was applied to determination of physical mechanical properties. The relief and surface structure at a submicronic and nanometer scale was investigated by means of the scanning nanohardness gage «NanoScan-3D». On the «NanoScan» base the method of measurement of the hardness, based on measurement and the analysis of dependence of loading at indentation of indenter in a material surface from depth of introduction of an indenter is realized. This method is a cornerstone of the standard of measurement of hardness ISO 14577. The
indenter of Berkovich type is applied to mechanical tests. It represents a trihedral diamond pyramid with a corner at top near 142°. The method of a measuring dynamic indentation consists in the following: the indenter is pressed into a sample surface with a constant speed, at the achievement of the set loading the indenter is taken away in the opposite direction. In the course of such test record of values of loading and shift of an indenter corresponding to it is made. The experimental curve typical for this method in the form of the schedule of dependence of loading (P) on depth of indentation (h) is presented in fig. 2. It consists of two parts corresponding to process of loading and unloading. Within this method
the hardness H of samples is defined by the equation:
p
,
here Ac - the area of a projection of a print at the maximum value of the enclosed loading Pmax.
Shift
Fig. 2 - The General view of a curve of loading, and the scheme of contact with designations of the sizes used in a method of calculation of the module of elasticity and hardness.
Creation of the image of a relief of a surface is carried out in a mode of a semi-contact scanning probe mikrosopiya. As a result of scanning the raster three-dimensional image is constructed.
(a) (b)
Fig. 3 - Relief of the third sample before processing (a) and after processing (a)
Experimental points were obtained with the help of a method of a measuring dynamic indentation. It is possible to describe a nature of change of properties due to such measurements. Diagrams of change of hardness depending on depth of penetration of plasmochemical gas for a firm alloy of tungsten-cobalt received as a result of carried-out tests are submitted in fig. 4, 5 and 6.
It is established that for the first sample there was a low-temperature degradation. Value of hardness decreased in the average by 56%. For an explanation of this effect the second experiment in the environment of the pure argon was made. The results showed that
hardness increased by 30%, at the same time the roughness also increased by 25%. It testifies the dispersion of the cobalt which is a part of an alloy, thus pure tungsten appeared on a surface, therefore there is an increase of hardness and roughness. For an exception of these factors plate voltage was reduced. It led (see fig. 6) to the increase of hardness by 60% and reduction of a roughness by 40%. Color of a product changed from characteristic metal blaze to yellowish turquoise. It also testifies film formation on an alloy surface.
Research of a roughness of a surface was conducted also by means of the nanohardness gage «NanoScan-3D».
Fig. 4 - Change of microhardness of a surface of an alloy: a - sample before processing, 6 - processed sample in Ar+C3H8, Q1=1500 cm3/min, Q2=1300 cm3/min, U=-20 V
Fig. 5 - Change of microhardness of a surface of an alloy: a - sample before processing, 6 - processed sample in Ar, Q=2000 cm3/min, U=-20 V
Fig. 6 - Change of microhardness of a surface of an alloy: a - sample before processing, б - processed sample in Ar+C3H8, Q^2000 cm3/min, Q2=1400 cm3/min, U=-20 V
The analysis of characteristics of the sealants processed in RF plasma discharge showed that physical mechanical values of apocessed samples possess the improved technological, operational indicators in comparison with control. the best result was obtaines in mix of argon and propane-butane gases in the ratio of 80% to 20%.
List of literary sources
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© V. Khristoliubova - Ph.D. student of plasma technology and nanotechnology of high molecular weight materials department, KNRTU, [email protected]; I. Abdullin - Ph.D., professor of plasma technology and nanotechnology of high molecular weight materials department, KNRTU, [email protected]; А. Khubatkhuzin - Ph.D., associate professor of plasma technology and nanotechnology of high molecular weight materials department, KNRTU, [email protected].
© В. И. Христолюбова - аспирант каф. плазмохимических и нанотехнологий высокомолекулярных материалов КНИТУ, [email protected]; И. Ш. Абдуллин - д.т.н., проф., зав. каф. плазмохимических и нанотехнологий высокомолекулярных материалов КНИТУ, [email protected]; А. А. Хубатхузин - к. т. н., доцент кафедры вакуумной техники электрофизических установок КНИТУ, [email protected].
