The study of microstructure of wear-resistant coatings applied for protection from abrasive wear of horizontal and tilt drilling drill bits

Drill bits of the cutting type over the period of their existence have undergone significant changes – from the use of carbide cutters to diamond composite PDC elements, in which the diamond layer is applied to a hardmetal substrate. Using such elements, it was possible to significantly increase the service life of the drill bits, however, during work, there is a significant abrasive deterioration of the bit body, which does not fully realize the advantages of PDC elements. Therefore, to protect the body from wear use special wear-resistant coatings. This work is devoted to research of microstructural coatings, namely coatings brands WokaDur NiA, HR-6750, HR-6750 with sublayer Rock Dur 47 on various steel substrates which applied by the gas-thermal spraying in Ltd “Oerlikon Metko Rus”. They were examined with the use of scanning electron microscopy, X-ray phase analysis and a Vickers micro-hardness tester. It was established that the microhardness of the coating matrix is 590–660 HV, and the microhardness of tungsten carbide particles reinforcing the coating, is 2145–2455 HV.


Introduction
During operation, the drill bit body experiences considerable abrasive wear in its contact zone with the cuttings of the drilled rock, which leads to a weakening of the fastening and premature removal of the PCD-elements from the bit body. Due to the incomplete development of the PCD-elements is significantly reduced resource of drill bit [1].
There are various methods for applying wear-resistant coatings, such methods include various variations of low-temperature CVD and PVD deposition or electrochemical and gas-thermal spraying that meet the following requirements for the application of a wear-resistant coating on the bit body [2][3][4][5][6].
Firstly, the coating must have a high hardness (higher than the hardness of the drill bit body, preferably above the hardness of the drilled rock) and corrosion resistance.
Secondly, there should be a close value of the thermal linear expansion coefficient (TLEC) of both applied coating and the drill bit body materials. If the condition is not met, then at high temperatures the coating will peel off.
Thirdly, the application temperature should not exceed the melting point of the drill bit body. The most commonly used wear-resistant coatings are applied by gas-plasma method such as hand-gasplasma welding. Therefore, the purpose of this work is a microstructural study of wear-resistant coatings containing tungsten carbide particles obtained by manual gas flame deposition in order to increase the resistance of the PDC-bit body to abrasive wear [7,8].

Materials and equipment
As substrates used samples were made from steels of marks St3, St45 and KHVG, which can be used as a drill bit body, and the coatings have been chosen as the following wear-resistant layer: -HR-6750 with 2.6 mm thickness, directly on the surface of steels; -HR-6750 with 2.6 mm thickness on a pre-applied Rock Dur 47 sub-layer with 0.2 mm thickness; -WokaDur NiA with 2.6 mm thickness directly on the surface of the used steels.
Complex study of the microstructure of wear-resistant coatings was carried out on a scanning microscope JSM-6480LV, located at the Center for Collective Use in NUST «MISiS».
X-ray diffraction (XRD) measurements were performed at NUST "MISiS" using DRON-3M (Co K α radiation). The micro-hardness of matrix and carbide particles was determined by Knoop/Vickers Hardness Tester the Tukon 1102, the load on the indenter was 200 g, the exposure time under load -5 s. Figure 1 shows the typical microstructure of the obtained coatings deposited on different substrates. A microscopic examination of the structure of the coatings is showed differences in all samples. Primarily due to the size of the carbide particles. Coating brand HR-6750 consists of particles with highly developed structure ( figure 1(b)). The basis of all coatings is a nickel matrix with additive elements (Cr, B, Si, C) and carbide particles. Elemental analysis of the matrices revealed the presence of the following elements and their quantitative ratio, which are summarized in table 1.

Results and discussion
The study of samples with a sublayer of Rock Dur 47 is showed partial dissolution in the process of applying HR-6750. As an example, figure 2 shows the distribution of elements on the surface of the thin section of the transition zone (coating-substrate) of the sample St45.
As a result of measurements of samples with different substrates and coatings, it was found that the microhardness of the matrix was 590-660 HV, and tungsten carbide particles 2145-2455 HV ( substrates of St3 and St45 steels, which is associated with partial hardening of the substrates from this steel when the samples are cooled after the application of wear-resistant coatings.

Conclusions
In the process of conducting microscopic studies of wear-resistant coatings used to protect the cases of diamond blade PDC-bits from abrasive wear during drilling, it has been established: the microhardness of the matrices of the coatings obtained is 590-660 HV, and the microhardness of the tungsten carbide particles is 2145-2455 HV; a micro-X-ray spectral study of the Rock Dur 47 sublayer revealed its partial dissolution in the process of surfacing a wear-resistant coating HR-6750.
Based on the conducted studies, we can recommend the use of WokaDur NiA as a wear-resistant coating, which is explained by the highest hardness of carbide inclusions in comparison with other coatings. As a substitute for WokaDur NiA coating, it may be recommended to coat the HR-6750 with a sublayer of the Rock Dur 47. Among the substrates, we can recommend structural carbon steel grade St45. Its mechanical properties satisfy the requirements imposed on the drill bit housing, and its quenching does not take place under the conditions of obtaining a wear-resistant coating.