Tribological properties of nanostructured boehmite
and chemical composition, and thermal properties of boehmite obtained by hydrothermal synthesis. Tribological properties, anti-friction, anti-wear and antiwelding properties were estimated. The possibility of using nanostructured boehmite for running-in of diesel engine was shown.
To date, many different additives are developed, which can be used in different periods of operation of the machines. In particular, additives are developed for running-in of the new or repaired assembly that maximize the fit of parts in the mates, optimization of roughness of working surfaces of parts and, as a result, minimization of friction and increasing of durability. Tribo-materials that reduce the coefficient of friction and wear, are used for initial period of the normal operation after repair of engines. For subsequent period of operation the repair-and-renewal compositions are developed, which restore the geometry of the parts in the zone of wear and compensate the increased clearances [1-3]. In the places of friction a composite layer is created under the influence of local temperature and pressure. This layer consists of products of decomposition of the additives, the material of the surface and oil. As a result, the formed gap decreases and the unit resource increases.
Many tribo-materials contain active natural components, which are heterogeneous in chemical and mineralogical composition, what leads to variation in tribological properties and may even cause the units failure. This forces developers to look artificial tribo-materials with stable properties . Such additives can be created based on monohydroxy aluminum (boehmite), an artificial material with stable properties. Like graphite and molybdenum disulfide, the boehmite has a layered structure, which effectively reduce the friction. In these compounds the connection is strong in the plane of the layer (covalent or ionic) and significantly less strong in the perpendicular plane (metallic, van Der Waals forces or hydrogen, as in the boehmite). This structure provides a sliding of the layers relative to each other and a low coefficient of friction. The basis of the structure of boehmite create alternating pairs of oxygen (internal) and hydroxyl (external) layers. The aluminum atoms in the crystal lattice of boehmite are surrounded by a deformed octahedral group of oxygen atoms. The studies showed the good potential of boehmite as a tribo-material .
The aim of this study was to estimate the structure, thermal and tribological properties of nanostructured boehmite obtained by hydrothermal synthesis, for different periods of operation of the engine.
Materials and methods
A powder of boehmite was used that was obtained by hydrothermal oxidation of aluminum powder . The microstructure of powder was determined using optical and scanning probe microscopes SolverNext (NT-MDT); the chemical composition was determined using atomic emission spectrometer with inductively-coupled plasma iCAP 6300MFCDuo (boehmite powder was dissolved in hydrofluoric acid); phase composition, crystallinity and size of crystals was determined using x-ray diffractometer Shimadzu XRD 6000 (Japan); specific surface area was determined by method of low-temperature nitrogen adsorption using Autosorb-1 device. Tribological studies were conducted using four-ball friction machine, a special device by Wagner, as well as 2070 SMT-1 and MTU friction machines.
Running-in of a new D-243 diesel engine was performed on a modernized test bench CI-3540-GOSNITI. Crankcase emissions was measured by the gas meter, continuously measuring fuel consumption. The control system allowed to register in time with a given counts increment the variation of speed of the motor shaft, torque on shaft, oil pressure in the engine lubrication system, oil temperature, water temperature in the cooling system of the engine, fuel consumption. Oil samples were taken to estimate the concentration of mechanical impurities during running-in without boehmite and with him four times: after three minutes from the start of cold running-in, after cold and after hot running-in, and after the test for durability. The Surtronic-3Р profilometer (Taylor Hobson, Denmark) was used to estimate the roughness of the cylindrical surfaces of piston rings, inserts of base and crankpin bearings before and after running-in.
Researches are conducted with use of only one engine. Before each test it was assembled with new parts of cylinder group, new upper head bushings and connecting rod inserts. In all cases, the boehmite was added into the oil with continuous rubbing by the method of serial dilutions.
Study of the structure, thermal and tribological properties of boehmite
Different batches of boehmite obtained by hydrothermal synthesis were used. Its distinguishing feature is the high degree of homogeneity, the stability of composition and structure in different batches. According to x-ray diffraction and petrographic analyses, in contrast to the powders of aluminum hydroxide, the boehmite of hydrothermal synthesis is well crystallized and consists of monohydrated aluminum AlOOH. Loss on ignition corresponds to the stoichiometric 15 wt.% (fig. 1). The synthesis process was modified to obtain powders of high purity. According to the spectral analysis, when using aluminum powder of high purity and deionized water, the impurity content does not exceed of 0.07 wt.%.
During heat treatment of boehmite, the release of water begins at 350-400 °C and is accompanied by the destruction of the crystal structure with the formation of anhydrous forms of aluminum oxide – gamma, delta, alpha and others (table.1). The observed increase in specific surface area caused by the destruction of structure and particle size reduction. Further reduction of the surface occurs as a result of sintering and recrystallization. According to petrographic and x-ray diffraction analysis, the transition into α-aluminum oxide (corundum) completes at 1300°C. Similar processes can occur in the contact area of friction, where temperatures reach hundreds of degrees and reach up to 1000°C. As a result, decomposition of the boehmite may occur on the friction surface with the formation of corundum. Mechanical stress in contact area can play an important role in phase transformations. The temperature rises also in the volume of metal around the zone of friction (measurements showed 70-100°C).
According to x-ray diffraction, coherent scattering area, which characterizes the size of the crystals of the initial powder, was 20-40 nm for different batches of powder and did not exceed 80 nm. Scanning microscope enabled to observe separate particles and aggregates of particles predominantly of 2-3 microns in size, and up to 10 microns. Thus, the boehmite of hydrothermal synthesis can be defined as a nanostructured material.
Research on four-ball friction machine showed that the introduction of 1% of the boehmite in the engine, industrial and rapeseed oil reduces the wear scar diameter of up to 34% at a temperature of 150°C, and at room temperature for certain oils (table.2).
Scuffing test using special device by Wagner showed that the introduction of boehmite improves the antiwear and antiwelding properties:
the time until the scuffing (stop) of the motor motor increased by 37 to 40%;
current consumption of the electric motor decreased by 15-20%;
the pressing force of the sample to counter until a complete stop of the motor increased by 40-50%;
the amount of wear of the sample decreased by 15-20%.
Tribological properties of greases with the boehmite as a thickener, were also determined using four-ball friction machine. Adding of 10% of a powder of boehmite has improved anti-wear properties (critical load increased from 863 N to 1570 N) and antiwelding properties (welding load increased from 1471 N to 1962 N, while the scuffing index increased from 43 to 62 units).
Using the 2070 SMT-1 friction machine the testing was performed according to the scheme of loading "roller-shoe". The shaft speed of the machine was 1000 min-1, the shoe was cut from a cylinder liner of the D-240 diesel engine. The test of the M-10DM engine oil with the addition of boehmite after the ultrasonic machining showed that 0.5% of the boehmite powder reduces the friction coefficient to 0.05, and the temperature in the friction zone – from 54 to 45°C.
The testing of the friction pair "plate-roller" (steel St10) was performed using the MTU friction machine. As can be seen from the table.3, addition of about 0.01% boehmite in the industrial oil I-20 reduces the wear of the roller in 5.9 times, and of the plate – in 5.6 times. In the solid oil at a concentration of about 9% the wear decreased, respectively, in 2.1 and 2.8 times. Corundum powder in the lubricant (concentration of about 2.4 %) has abrasive properties, which leads to increased wear.
Comparative tests of the up-to-date tribo-materials has shown that the most effective are multi-component formulations containing, in addition to the mixture of minerals, organometallic surface-active agents (surfactants). Nanostructured boehmite in the composition with surfactant after ultrasonic treatment increased the load capacity of the pair, in comparison with the oil, in more than 2 times and in 1.8 times compared with the composition containing boehmite without the ultrasonic treatment and surfactants. As a result, in the pressure range from 200 N to 550 N the coefficient of friction decreased to 0.044-0.05 (fig.2).
Ultrasonic treatment reduces size of aggregates of boehmite and improves the stability of the suspension. Surfactant also contributes to aggregate stability. Without this treatment and surfactants can occur stratification of the suspension during storage. Low coefficient of friction and low wear (determined by the length of the hole of the wear) suggest long life of friction units. Nanostructured boehmite additive improved the efficiency of the investigated industrial tribo-materials.
Running-in of the D-243 diesel engine with use of boehmite showed stabilization of the compression in the cylinders at the level of 3.0 MPa after 30-40 min. Without boehmite for these values of compression was required not less than 80 min (fig.3). The roughness of the liners and piston rings after running-in without boehmite is greater than with him in 1.38-1.65 times (fig.4). Content of mechanical impurities in oil is shown in fig.5, and the crankcase emissions – in fig.6.
Experiments show that after the cold running-in with the boehmite total wear of mates of the diesel is 34% higher than when running without boehmite, but the total wear during the entire running-in with boehmite is 5.8% less. The calculation of the running time required to increase the content of mechanical impurities in oil by one percent, showed improvement in the wear resistance of the mating of the engine by 22% (27.77 h/% while running-in without additives to 35.71 h/%). In general, the addition of the boehmite increased the durability of diesel and reduced running-in wear. After 60 min running-in the wear of the first compression ring with the use of boehmite was less in 2 times, the crankcase emissions – less in 1.6 times, and the oil temperature – less by 15-20°C.
The maximum effective power of diesel after 90 min. of running-in with use of oil with boehmite was 52.5 kW, and specific fuel consumption was 257 g/l, what is close to the performance of a diesel engine after 50-100 hours of work. And after 90 min. of running-in with use of oil without additives the maximum power was about 45 kW, and the specific fuel consumption was about 270 g/l. After 120 min. the wear of the first compression ring was less in 2.5 times, and crankcase emissions – less by 12.7 %.
Further, the diesel engine was running with variable load during 70 h: 30 h with increasing load, the rest of the time with load of 50 kW at 1800 rpm. The test showed no significant difference in wear of top compression rings in both cases. The average crankcase emissions when using boehmite was 11.0 l/min, and when using a conventional oil – 12.6 l/min.
In general, the use of boehmite decreased the time of the full running-in of D-243 diesel engine in 1.8-2 times, initial wear of the first compression ring in 2.5 times, the flow of crankcase emissions by 12.7%, oil consumption by 27 %.
Taking into account the knowledge about tribology, the role of the boehmite in an environment with standard greases can be summarized as follows:
grinding of the friction surfaces, improving surface smoothness, reducing the mechanical component of the friction coefficient;
cleaning of friction surfaces, removing oxide films and defect structures. This ensures access of the boehmite particles to catalytically active metal surface and accelerates the formation of anti-friction coatings;
adsorption on the surface of the boehmite particles of resinous substances. Created particles separate the parts ("third body" in tribosystem) and reduce the coefficient of friction.
Study the structure, thermal and tribological properties of boehmite of hydrothermal synthesis, which has a nanocrystalline structure, high phase and chemical purity, has shown that it has anti-friction, anti-wear and antiwelding properties. In compositions with surfactants and after ultrasonic treatment it increases the load capacity of the pair in more than two times, reduces the friction coefficient to 0.044-0.055.
The addition of boehmite up to two times accelerates and improves the quality of engines running-in. Recommendations are developed for running-in of engines using nanostructured boehmite.
The obtained results indicate the prospects of further research in this area and of the conducting of laboratory, bench and operational tests.
The study was performed with financial support of Ministry of Education and Science of the Russian Federation (agreement No.14.613.21.0004 from 22.08.2014, unique identifier of the project RFMEFI61314X0004).