The basics of preparation technology of titanium triboengineering oxide coatings and their tests

The paper describes the developed basics of the preparation technology of solid ceramic coatings on titanium and its alloys. The coatings have been formed by the method of microplasma surface oxidation in electrolytes of different composition. The authors have determined the modes of wear-resistant coating application on titanium alloy, which have significantly decreased coating porosity and made it possible to increase its wear resistance. They have also carried out physical and mechanical, as well as tribological tests of the materials obtained and formed a database of their actual properties. The paper shows that self-lubricating coatings can be operated for a long time at moderate loads and sliding speeds in friction units without using a lubricant. It also describes a newly developed and improved existing equipment to create coatings and study their tribological properties.


Introduction
High specific strength and corrosion resistance of titanium and its alloys determine their wide applicability in engineering and medicine [1,2].However, the use of titanium alloys in friction units is not so common due to their low wear resistance.Specialists paid much attention to studying wear resistance of titanium alloys.In particular, it is shown that wear resistance increase can be obtained by nitriding, carburizing, boronizing.However, these methods are not always acceptable.The created surface layers are extremely brittle.High temperatures and long processing times lead to grain growth and lower strength properties.The analysis of the research results in recent years implies that traditional methods for improving wear resistance of products made of titanium and its alloys are ineffective.
A promising direction for improving titanium surface properties that are important in friction is a creation of a protective oxide layer on its surface using the microarc oxidation method [3][4][5][6].The microarc oxidation method is the following: when high-density current is passing through the valve metal-electrolyte interface, it creates conditions for appearing microplasma discharges on a metal surface in microvolumes with high local temperatures and pressures.The result of discharge electrochemical action is a formation of a surface layer consisting of oxidized forms of metal elements of the base and electrolyte components.Depending on the choice of a microarc oxidation mode and electrolyte composition, it is possible to obtain ceramic coatings with a wide range of physical and mechanical properties [7][8][9].The coating structure obtained by the microarc oxidation method is porous ceramics with a complex structure, which is formed due to metal surface oxidation and inclusion of electrolyte elements into the coating composition.Studies in this field made it possible to obtain oxide layers several micrometers thick on titanium.However, coating properties haven't been optimized for dry friction.
The research objective was to obtain wear-resistant coatings on titanium using the microarc oxidation method and to study the tribological properties of coatings during friction without a lubricant.

Methodical research issues
The coating synthesis facility using the microarc oxidation method consisted of a capacitor-type current source and a cooling jacket bath filled with a test electrolyte.The studies on obtaining a titanium coating have covered four electrolytes: l) NaOH + Na2O(SiO2)n + Н2О; 2) NaF + Na2B4O7 + + Н2О; 3) НзРО4+ Н2О; 4) H2SO4 + Н3РО4 +Н2О, that differed in passivating, dissolving and conductive properties.
In order to test the obtained anti-friction composite coatings, the counter samples of KhVG hardened alloy tool steel (GOST 5950-73) with a hardness of 45-50 HRC were used.Triboengineering tests of oxide coatings have been carried out on an MTP friction machine [10].The friction scheme is disk-finger.

Research results
The presented electrolytes are combined, except the third one.Therefore, they cannot be attributed to any of the groups of electrolytes in the literature.The choice of electrolyte is a selection of the optimal ratio of dissolving and passivating properties.So, for example figure 1 shows that the first electrolyte (curve 1) has weak passivating properties and the insufficient dissolving action for a titanium alloy at the same time.This leads to a slow sparking mode process output and further unstable flow.The second electrolyte has better passivating properties compared with the first one.However, they are also insufficient for making a dense coating that has good adhesion to the titanium alloy surface due to the coating growth deep into the coated metal.The third electrolyte has good passivating properties, but a weak dissolving ability.This leads to an increase in a dense wear-resistant coating that does not have sufficient adhesion to the coated sample surface, which is determined by local coating stripping sites.The fourth electrolyte has been selected due to the recommendations in the literature.This electrolyte has good passivating, dissolving properties, and electrical conductivity.The resulting wearresistant coating had sufficient thickness (more than 10 μm) and good adhesion.However, it had high porosity (30-50%), which significantly increased its brittleness and reduced effective hardness.There is a determined percentage ratio of electrolyte components (2.2% H2SO4 + 2% H3PO4) and current coating application modes (current density 30 A/dm 2 ), at which it was possible to reduce the porosity to 30% on the coating surface and to 15% in the middle layer due to reducing the number and size of pores (figure 2).An increase in the current density has allowed increasing the temperature in the operating zone.It has caused melting and tightening of pores.The electrolyte composition has reduced the microarc area and, thus, reduced the size of the pores they burn.The coating obtained under these conditions has low roughness of the order of Ra 0.15-0.30μm.Therefore, it can be used in tribocoupling details without additional finishing of a coating.The properties of the obtained titanium coatings are given in table 1 (Ukon is a final voltage measured before the process stops).The relatively low microhardness is the result of a titanium dioxide in the crystal anatase modification.The tests revealed the superiority of triboengineering properties of the optimized fourth electrolyte.This made it possible to confirm its wear resistance increase under dry friction conditions by structure improving that decreased the number of stress concentrators and increased coating resistance to fatigue wear.It is obvious due to the increased number of operation cycles with the same loading conditions.
Figure 3 shows the graphical view of coating tribo properties on the fourth electrolyte.The dependence presented in figure 3(a) shows a run-in coating area during the first 3÷4 operation hours defined by a decrease in friction force.Then friction coefficient increases after changing mechanical and physicochemical properties of the coating in thickness and the influence of an underlying metal due to reducing of coating thickness.
Curve 1 in figure 3(b) shows the dependence of a linear wear intensity on a contact pressure magnitude for the coating obtained according to the technology described in the literature, and with 30-50% porosity.Curve 2 shows the dependence of a linear wear intensity on a contact pressure magnitude for the coating obtained according to the improved technology, and with 15-30% porosity.This proves an increase in coating wear resistance due to increasing effective hardness and improving its adhesive properties.The results at different sliding speeds within the abovementioned interval differ within a measurement error.When a contact pressure is 1.5 MPa, the linear wear intensity is of the order of 8•10 -9 .

Conclusion
The paper proposes the basics of the preparation technology for wear-resistant coatings on titanium parts for tribounits that function under lubricant shortage conditions.It also determines the percentage ratio of electrolyte components and current modes of applying a wear-resistant coating on titanium alloy when the porosity decreased to 30% on the coating surface and to 15% in the middle layer, which made it possible to increase its wear resistance.The resulting titanium alloy coating differs from the known low roughness of the order of Ra 0.15-0.25 μm.Therefore, it might be used in friction units without finishing treatment.

Figure 1 .
Figure 1.The dependence of current on voltage when forming the VT1-0 titanium alloy coating in four electrolyte compositions.

Figure 2 .
The microstructure of coating surfaces (×1750) that have been obtained with different oxidation modes: a is a coating obtained by a known technology (30-50% porosity), b is a coating obtained by an improved technology (15-30% porosity).

Figure 3 .
Figure 3.The dependencies of a friction coefficient (a) (contact pressure is 1.5 MPa, sliding speed is 0.47 mps) and wear intensity (b) of titanium coatings on time and contact pressure for the fourth electrolyte.

Table 1 .
The properties of titanium coatings.