Lubricant composition to improve abrasive materials processing
Selection of the optimal composition of the SLP, to stabilize the cutting ability of the grinding wheel, is a very challenging task. Components of the SLP must meet the following requirements: functionality (lubricating ability), storage stability, fire safety, environmental safety. The use of less reactive components instead of environmentally hazardous ones, and of cheap fillers, the effectiveness of which can be improved by the use of combinations of additives of nanomaterials and relatively cheap organic materials, determines the possibility of obtaining SLP that meets all the requirements listed above. The authors have developed, patented and tested a SLP with fillers of nanomaterials, which enhances the efficiency of processing of metals and allows achieving of high quality surface treatment, in particular, of significant reduction in its roughness.
The authors task was to develop a lubricant composition for abrasive machining of various metals and alloys, allowing to achieve the highest surface quality due to a significant roughness reduction. According to sanitary-hygienic requirements to modern SLPs were taken into account the disadvantages of the known solid lubricants containing environmentally harmful components: tin, lead, cadmium, etc.
During cutting or grinding in the contact zone produces a lot of heat and the temperature reaches of 1000-1600 °C . With heating the workpiece on its surface form burn marks. Thus, in the manufacture of solid lubricants must also consider the need for cooling the machined surface. Reducing the coefficient of friction between the sanding surface and an abrasive wheel is also relevant , since the friction in machining processes has a great influence on the tool life. This problem is solved by adding surface-active substances (surfactants), which have a great affinity with metal (wetting) and, therefore, reduce friction .
Solid lubricants are used to improve process efficiency of abrasive machining processes. Superfine abrasive fillers provide formation on the friction surfaces of the structure with high tribological characteristics. The influence of nano-additives on tribological processes at the points of contact have been studied in many works [4, 5]. Thus, the Nanotech R&D production facility has developed a number of materials used as additives for motor and industrial oils. These include a cluster diamonds with sizes up to 100 angstroms and ultrafine nano-diamond-graphite. In  was established that the introduction of nanopowders of copper and nickel in motor oil SAE10W-30 reduces wear and friction of lining and the shaft, which in turn affects the temperature.
A distinctive feature of the solid lubricant, developed by the authors, is content in its composition in addition to high-performance surfactants (mineral oils, low molecular weight polyethylene, etc.) of the mixture of ultrafine powders of carbothermic reduction of leucoxene (product of enrichment of oil-bearing silicon-titanium ores), and also of aluminum nitride (AlN). The latter is a technical ceramic material, which has a very interesting combination of very high thermal conductivity and excellent insulation properties.
About 90% leucoxene components are oxides of titanium and silicon (table.1). Carbothermic reduction of leucoxene at certain temperatures in a gaseous environment allows to obtain mechanical powder mixture of silicon carbide and titanium carbonitride with different ratio of components (50-70 wt.% TiC1-xNx and 20-40 wt.% SiC) .
The specific surface area is one of the most important characteristics of the nanomaterial, which determines the activity of physico-chemical processes and many technological properties. Information about thiso parameter is particularly necessary in the case when the surface works in the process. The most important characteristic of nanomaterials is also the particle size.
Powder of aluminum nitride used to create the SLP had a specific surface area of 27 m2/g. The particle size analysis by laser diffraction using Horiba LA-950 device with ultrasound treatment showed that the average particle size is 0.15 μm. Due to the high fragility of recycled leucoxene grinding in a ball mill allows to obtain powders of different particle size. In our case, the average particle size was 3–5um.
Correctly matched binder improves adhesion of the workpiece to the material, promotes the formation of a uniform lubricant layer on the surfaces of friction, improves durability of the SLP. A binder having a sufficiently good strength of adhesion to the surfaces of friction, greatly enhance the wear resistance of the SLP. It is established that anti-friction and adhesion properties of the SLP essentially depend on the content ratio of the filler and binder: the more binder, the stronger is the link of the films generated in the grinding process with the surface; the less binder, the higher are the frictional properties of the SLP. The boundary values of the component content in the lubricant are selected in accordance with the experimental data.
Conducted by the authors experimental studies revealed that the additional introduction in the lubricant of linoleic acid can improve the lubricating properties. Thus, when the content of linoleic acid is less than 16.5 wt.% there is an increase of friction, and when the content of linoleic acid is more than 31 wt.% there is slippage of the tool on the material and, consequently, increasing of the processing time. It was also experimentally determined the optimum composition of the filler (finely dispersed powder). The best properties were obtained with the introduction of the product of thermal reduction of leucoxene and of aluminum nitride in the ratio 1:2.
A series of experiments was performed to determine the optimum ratio in the composition of the filler of "hard" and "soft" components: aluminum nitride, which has high hardness, ensuring the removal of roughness from the surface, and the product of thermal reduction of leucoxene, which provides smoothing of the irregularities resulting from the actions of the "hard" component, and the polishing of surface. Only a certain proportion ensures the achievement of the technical result – the reduction of roughness of the processed surface. Thus, when the ratio of the product of thermal reduction of leucoxene and aluminum nitride is less than 1:2, cleanliness of the machined surface deteriorates, and the ratio more than 1:1 leads to clogging of the working surface of the abrasive tool.
The use of ultrafine AlN powder leads to the rapid formation of the cladding. The combination of leucoxene and aluminum nitride provides excellent synergistic effect and the cladding film will be very hard (resistant to wear) composite with thickness of 4 μm. Thus, the contact temperature in the contact zone reduces to 30%, which leads to reduce of the probability of burn marks and cracks in the surface of sanding details.
Making and test of the SLP
As initial components were used stearic acid (GOST 6484-64), linoleic acid (GOST 30623-98), low molecular weight polyethylene (NMPA-1, TU-6-05-18-37-82), spindle mineral oil (GOST 1642-75), chloro-fluorocarbon oil (OST 6-02-6-81) and a mixture of ultrafine powders of product of thermal reduction of leucoxene and aluminum nitride, taken in the ratio 1:2. In a melt of a mixture of stearic and linoleic acids at 70-75°C adds a melt of low molecular weight polyethylene and emulsion of chloro-fluorocarbon oil in mineral oil. Then adds a mixture of ultrafine or micron powder of product of thermal reduction of leucoxene and aluminum nitride. The resulting mass is stirred for 10-15 minutes at 70-75°C. The device for the synthesis of the SLP is presented in fig.1.
The resulting lubricant was tested when grinding the samples of stainless steel, copper and titanium alloy. Samples with a size of 18×30×40 mm was polished by 14А25Н belt (TU 2-036-766-78) with the pressure of 29.4 N and a speed of 25 m/s. Burn marks was controlled visually with a microscope by the appearance of colours. The surface roughness Ra was measured on the profilograph-profilometer mod.201. The lubricant was applied every 10 cycles. The roughness Ra data of the treated surface when using the SLP is shown in table.2.
The SLPs (fig.2 and 3) were tested and applied in the following companies: "Ural Works of Civil Aviation", "Ural Turbine Works", "Sverdlovsk tool plant", etc. The results of tests are confirmed by relevant documents. The compositions of the solid lubricating are protected by laws of Russian Federation.
The effects of the application of the SLP are achieved thanks to the unique properties of the surfactants contained in the lubricant composition, properties of finely abrasive materials and technology of its production. The technical results of the development are the reduction of roughness in grinding and in polishing, elimination of burn marks, the reduction of temperature in the contact zone and the shortening of the time of surface treatment. Solid lubricant contains no harmful ingredients and is environmentally friendly. The service life of the abrasive tool (due to keeping the resistance of profile and a significant reduction in "clogging") increases by 10 times and more in comparison with previously used lubricants.