Cutting ceramic inserts : the influence of abrasive machining and surface coatings on the operational characteristics

Ceramic cutting tools have a large potential by high speed processing of difficult-to-cut steels and alloys, however due to its fragility they don’t assure the required reliability level of cutting process. For improving the operational characteristics of the ceramic cutting tool combined treatment can be used namely the preliminary planetary grinding of the insert surface and the following deposition of the vacuumplasma coatings (TiCr)N, (ZrCrHf)N and other.


Introduction
Currently various tool ceramics are widely used for high-speed processing of difficult-to-cut steels and alloys [1].These materials have high hardness, heat resistance and are characterized by low tendency to adhesion interaction with the processed materials.Despite the aforementioned advantages, industrial use of tools equipped with ceramic cutting inserts is still limited because of its low reliability due to breakoff and chipping of the cutting head in different periods of operation.The main reasons for this are low strength and thermal conductivity of ceramics and various volumetric and surface defects (microcracks, pores, the tensile stresses generated during grinding of the insert) [2][3][4].
Two different processes were investigated to determine the influence of the abrasive treatment condition a Corresponding author: m.volosova@stankin.ru of cutting surfaces on reliability of ceramic inserts during operation: flat grinding and planetary grinding.After abrasive treatment the following vacuum-plasma wearresistant coatings ZrN, (TiCr)N and (ZrHfCr)N were deposited on the ceramic inserts.Subsequently the ceramic samples were used for integrated metallophysical studies: evaluating the surface roughness, the residual stress in the surface layer, the bend strength and for performance tests.
The main purpose of this paper is to establish optimal conditions of abrasive treatment and subsequent vacuumplasma wear-resistant coatings deposition on ceramic cutting insert providing improved operational performance based on the results of theoretical and experimental studies (for example the treatment of bearing steel).
Manufacturing of ceramic inserts includes a large number of operations and one of the most important operations is an abrasive treatment.It is well known that the process of tool ceramics diamond grinding is accompanied by high contact temperatures and power loads that result in the formation of the surface defect layer containing a significant amount of micro-cracks and pores as well as fields of tensile stresses [1,5,6].This has a very negative impact on the operational functionality of ceramic inserts and along with low mechanical strength and it is the main reason for their brittle fracture during the cutting.Currently more efficient technologies of diamond machining of ceramics' are developed with reduction of processing thermal stress.The traditional and well-studied approach to solve this problem is optimizing the characteristics of a diamond tool and grinding modes [1,7].Until now, only the influence of aimed change of the kinematics of the R e t r a c t e d grinding wheel motion or processed inserts motion on the output characteristics of the grinding process is practically unexplored.

Experimental techniques
For implementation of the research two types of ceramics with greatly differing properties were chosen: oxide ceramics based on Al 2 O 3 , which has in its composition the additives of TiC and ZrO 2 providing a higher viscosity and nitride ceramics based on silicon nitride containing more than 94% (vol.)β-Si 3 N 4 .In the study of mechanical properties, the ceramic rectangular samples (50 × 5 × 4 mm) were used.Then in performance tests by turning of hardened steel the square-shaped inserts (12.7 × 4.76 mm) with mechanical fastening were used.
Analysis of the planetary scheme of flat grinding demonstrated considerable reserves of reducing the process thermal stress and consequently reducing the number of defects in the surface layer of the ceramic inserts.The kinematics of the planetary grinding allows implementing complex ways of reducing the process thermal stress: the interrupted grinding, comparability of grinding wheel motion speed and workpiece motion speed, the work of abrasive grains with different faces.These actions help to implement more efficient use of the cutting ability of grains and disposal of grinding waste.
For evaluation of the effectiveness of using of planetary grinding instead of conventional face grinding, comparative analysis of the processed surface roughness and residual stresses in the surface layer was conducted.In the experiments modes of planetary grinding process and face grinding process have been widely varied, the characteristics of diamond wheels left unchanged (wheels made from synthetic diamond AC6 with grain size 80/63 and ceramic bond K5 have been used).
The planetary grinding scheme was implemented on Duomat ZL-500 machine (HAHN and KOLB Company, Germany).The machine is equipped with two grinding wheels and planetary head consisting of five holders of ceramic inserts (satellites) that revolve around their axis and the axis of the planetary head (Fig. 1).The upper grinding wheel had been effecting on ceramic inserts with a constant pressure p ≈ 10 Pa.The process of face grinding was also implemented in the framework of the scheme with grinding wheel constantly pressing on ceramic insert processed surface to ensure the accuracy of the collection of experimental data.
Distribution of residual stresses in the surface layer of the ceramic samples after various schemes of planetary and face grinding were examined by diffractometer (Huber company, Germany) with the help of radiographic examination.Similar pattern was identified in the stress generated by the defective layers of nitride and oxide ceramics, but their values turned out to be significantly different (Fig. 2).
Large compressive stresses were formed in the surface during face grinding then its sharp decrease took place and there was a transition to the tensile stresses, which gradually decreased and stabilized.When using a planetary grinding scheme 2-3 times smaller compressive stresses were formed and tensile stress did not exceed 100 MPa.In case of using the planetary grinding, the defect layer depth did not exceed ∼4 mm while in case of using the face grinding it was about ∼6-8 mm.
The roughness of the ceramic samples surface after grinding was determined on the Talysurf 120 automated profilometer (Taylor Hobson, UK).The measurement was made by probing the sample surface with diamond stylus.Profilogram of irregularities profile in the Cartesian coordinate system was recorded by electrothermal method.

Results and discussion
Studies of ceramic samples surface roughness showed that there are certain dependencies between the values of Ra and conditions of the planetary grinding (Fig. 3).With the increasing of grinding wheel motion speed the surface roughness decreases.It is apparently because of thickness of cut, attributable to a single diamond grain, decreases.However, this positive trend was observed only in the speed range 15-35 m/sec.Studies have shown that increase in speed above 35 m/sec greatly increases the thermal stresses of grinding process and leads to chipping at the edges of the inserts and their cracking.
Another important feature was found during the investigation process of planetary grinding: during the grinding with constant pressure the as-trued wheel deeply takes root into a surface of the ceramic workpieces forming a high rough relief, while during its dulling and reducing the thickness of cut, the roughness height can be reduced to 2.5 times (Fig. 4).
Comparison of insert surface roughness, which can be reached with the help of planetary and face grinding under recommended conditions [5,6], shows that planetary grinding helps to reduce the roughness parameter Ra up to 1.3 times.
The role of the subsequent deposition of wear-resistant coatings on ceramics is highly controversial.For example, the coating may provide effect of healing of defects generated in a surface layer of ceramic inserts at different stages of production (mainly during diamond grinding) [8,9].Moreover, the coating on the surface can slow down or halt the development of edge cracks extending from the bottom layer towards the surface.
The selection of design and composition of wearresistant coatings for ceramic inserts is an ambiguous task.Three different coatings (Zr, Hf, Cr)N, (Ti, Cr)N and ZrN were proposed for deposition on ceramics based on Si 3 N 4 and Al 2 O 3 .Modes of deposition of these coatings are given in Table 1.Each of these coatings has specific features that can significantly improve the performance of ceramics in various operation conditions [10][11][12][13][14][15].
(Zr, Hf, Cr)N-coating has high thermodynamic stability and can maintain high hardness under conditions of high cutting temperatures.All three elements included in the coating have significant defects in the inner electron orbits, which make them highly susceptible to the formation of couplings with atoms of nitrogen.Moreover, this coating has the same change of linear expansion coefficient during heating as ceramics, which also allows getting the high performance of the coating.
(Ti, Cr)N-coating has a high resistance to adhesion processes and sufficiently high ductility.Important advantage of this coating is the low cost of its raw materials.ZrN-coating has a sufficiently high thermodynamic stability and resistance to the initiation and growth of cracks, due to the high binding energy in the crystal lattice of the coating.Another characteristic feature of the ZrN coating is a small amount of drop phase.

R e t r a c t e d
As results of studies it was shown that the use of planetary grinding and the subsequent (ZrHfCr)N-coating deposition can significantly increase the flexural strength of ceramic inserts.Table 2 shows the results of strength measurement conducted with the help of four-point bending of samples, which were processed in different modes of surface treatment.The average strength was calculated from the results of testing of 20 samples from four different groups.
Table 2 shows that the average bending strength of samples after only planetary grinding is slightly increased compared with the initial samples while the coating deposition helps to increase this value for the oxide and nitride ceramics by more than 10%.These results suggest that planetary grinding as an independent operation did not significantly improve the strength characteristics of the ceramic inserts, but in combination with wear-resistant coating, it allowed to achieve significant results: up to 22% increase of oxide ceramics strength and up to 18% increase of the nitride ceramics strength.Moreover it somewhat reduces the scatter values.
On the last stage of the studies, the comparative tests of square-shaped oxide ceramics inserts used in the bearing industry for finishing and machining of hardened steels and the same inserts that were subjected to the surface treatment in accordance with the proposed technology (a planetary grinding of surface and wear-resistant coating deposition) were conducted.
It is well known that parts of bearing work under multiple loading.The surfaces under concentrated load are very small and having high (about 3000-5000 MPa) variable local stresses which are distributed to a considerable depth and eventually lead to fatigue failure of bearing.
Fatigue failure starts with the formation of microcracks and their subsequent spreading deep into the material.Formation and development of fatigue cracks occur in the most intense microvolumes of bearing metal, which are damaged by surface and subsurface stress concentrators.Seat of destruction might have defective metallurgical and technological nature; the defects of the second type are laid on the operations of heat treatment and machining.Metallurgical and technological defects can be fracture nuclei.The defects of the second type are laid on the operations of heat treatment and machining [16][17][18].
During manufacturing of bearing large parts (primarily, racers) substantial deformations take place during their heat treatment, so it is necessary to assign a large allowance for grinding (up to 2 mm per side).Such large allowances lead to considerable complexity of the grinding operation, as well as to increased consumption of abrasive tools.At the same time, the grinding operation itself is one of the main sources of technological defects that are formed on the surface of the component local tempering spots (burns).
The tests were carried out on CNC lathe for turning the outer diameter of the inner racer made from hardened steel (62-65HRC).Square-shaped inserts with γ = −10 • and α = 10 • have been used.
Table 3 shows the test modes and results of their implementation which show that inserts subjected to a combined surface treatment not only exhibit higher resistance, but also have higher operational reliability.The wear kinetics once again confirms high reliability and low wear rate (Fig. 5).

Conclusions
The following conclusions can be derived.

Investigation of the work conditions and the causes
of the ceramic tool failure by processing of hardened steel allowed developing the technology of improving the quality of the insert's surface layer.The technology consists in the planetary diamond grinding and subsequent deposition of vacuum-plasma coating.2. Comparative studies of quality of the ceramic insert surface layer after the planetary grinding and the conventional face grinding revealed that with the help of planetary grinding significant reduction of residual stresses in the surface layer and reduction of processed surface roughness can be reached.3.With the help of four-point bending method, it was found that using a planetary grinding in combination with the deposition of wear-resistant coatings can significantly increase the average bending strength oxide and nitride ceramic samples (up to 22% to increase durability of oxide ceramic and up to 18% of nitride ceramics and also to reduces dispersion of its values).4.After test results processing, it was found that the developed principles' application of surface treatment improves the resistance of ceramic inserts during the processing of hardened steels.Furthermore, it was found that the surface treatment significantly improves the reliability of the cutting process: during the turning operation up to 90% of failures are reduced to failures related to the fixed value of flank wear.Thus during the machining of bearing parts it can be recommended to replace grinding tools with turning tools equipped with ceramic inserts.

2 2 Fig. 3 .
Fig. 3.The influence of planetary grinding modes on surface roughness Ra of nitride ceramics insert.

Fig. 4 .Table 1 .
Fig. 4. The profilogram taken from the oxide ceramics sample surface after planetary grinding process.

705-page 5 R e t r a c t e dFig. 5 . 3 .
Fig. 5. Wear kinetics of ceramic inserts during the processing of racer, made from hardened steel: (a) ceramics sample without wear; (b) ceramics sample after 15 min of operation; (c) ceramics sample after 25 min of operation.

Table 2 .
Flexural strength of inserts after different types of surface treatment.