The adhesion solidity, physico-mechanical and tribological properties of the coating of titanium nitride

Influence of variable technological factors (arch current, fractional pressure of gas in the camera) on structure, physic-mechanical and tribological features of an ion-plasma coating of titanium nitride has been investigated. The adhesion solidity has been put to the test and the mechanism of destruction of a covering has been also researched by a skretch-test method. The optimal mode of spraying at which the formation of the nanostructured bar coating of TiN has been defined. The covering offers an optimal combination of physic-mechanical, tribological and solidity features.


Introduction
Nowadays, when abrasion of basic means of production and transport facilities in Russia reaches 80 %, the main task is to increase reliability and operability of various details of clusters and mechanisms in machine-building branch. In addition, increase of reliability and resource of further developed technique is also relevant.
In spite of the fact that thin coverings on the basis of refractory nitrides of the transitional metals are rather well studied and are widely used in many areas of human activity, scientific and practical interest in them does not fade away for the specific physic-mechanical propertieshigh hardness, a low friction coefficient, endurance, resistance to corrosion, etc. [1,2].

Experimental technique
The ion-plasma coating of TiN was applied on the VU-2MBS installation on small-size thin-walled exemplars of steel on the modes given in the table 1. The spraying of coating was realized in definite periods of time of processing in one production cycle: cleaning and activation of a surface of details with method of ionic bombing Ti + in a periodic mode: 5 s (processing) + 5 s (pause), etc.; drawing an underlayer α-Ti -10 min; TiN deposition -50 min. The microstructure of the coating of TiN was studied with a submicroscopy method on a raster microscope of VEGA//TESKAN.
Microhardness (H μ ) of compositions "covering + basis" was measured on PMH-3 microhardness, theoretical calculations of the true microhardness of a coating were made according to the formula [3] that takes into account the influence of a substrate on measurement process: , where H ca microhardness of a coating; H coma composition microhardness; H ba substrate microhardness; tthickness of a coating; hdimpling depth.
Research of frictional features was carried out using laboratory installation of friction at dry sliding at loading 1 N on an indentor of sapphire.
Tests of endurance was carried out at the Echo installation at dry transversal contact of the rotating core (steel U10, HRC = 65) by diameter ~8 mm load of 3 N during various time. Endurance of the original material and coating of TiN was estimated in geometrical parameterdiameter of a spot of the depreciation.
Process of destruction and the adhesion solidity are analysed by a sclerometric method by Revetest RST skretch-tester. Tests were passed with use of a diamond spherical indentor. Test parameters: the growing load of the indentor (AL)from 1 to 50 N, the speed of movement of the indentor -1 mm/min, length of scratch (L) -3 mm, the loading rate -16.33 N/min, sensitivity of the acoustic emission -5.

Results of the research
By method of a submicroscopy it has been ascertained that at all modes of spraying the coating with the bar structure of TiN grains (figure 1) is received. The tendency of formation of the nanostructured TiN grains with decrease of pressure of reactionary gas in the camera and current of an arch (figure 2(a)) has been revealed. The received dependences of modification of a microhardness of a coating TiN on arch current at different fractional pressure of gas in the camera showed that the maximal value H µ 50 ~19.4 GPa of a covering is received at arch current of I = 120 A and is caused by the finely divided microstructure. Decrease of microhardness of a coating by increasing the grain size ( figure 2(b)) is installed with increase of arch current and gas pressure in the camera. Application of the nitride coating promotes decrease of the friction coefficient of the processed surface. Spraying of the TiN coating at the "U = 140 V, P = 0.04 Pa, I = 120 A" mode leads to decrease of a friction coefficient of an effective area of a steel detail in ~1.4 times (figure 2(c)).
It has been ascertained that the titanium nitride coating received at arch current of 120 A (figure 2(d)) have the greatest endurance (in ~4 times).
Analyzing results of the adhesion tests it is possible to mark out various threshold values of ultimate load leading to various types of destruction. Three stages of destruction are typical for coating with a microhardness of 13 GPa (figure 3) and two stages are typical for coating with a microhardness ≥15 GPas ( figure 4).
At the first stage (load of an indentor to ~13 N) the indentor penetrates monotonous into a coating, at the same time the frictional force poorly increases, and amplitude of an acoustic emission remains invariable. The indentor leaves smooth slight marks on a coating. Sliding of a diamond indentor on a covering takes place with very low friction coefficient.
At the second stage an increase in amplitude of an acoustic emission and change of an inclination of curves of a frictional force and friction coefficient take place. At scratching with loadings more than 13-15 N chevron cracks appear at the bottom of scratch, cleavage of separate flakes (a cohesion destruction of a covering occurs) and islet detachment is observed on edges of scratches. Increase in loading over 20 N leads at first to the local and then permanent cleaving of a covering. The permanent cleaving of a covering leads to growth of amplitude of a signal of an acoustic emission, a frictional force increase monotonously up to 10 N. At further increase in a frictional force a fast attrition of a coating occurs.