Modeling and Simulation for Material Selection and Mechanical Design Part 12 docx

While the hardness and heat resistance values of the HSS T15 and DCPM are similar, the wear resistance of the latter is significantly higher Table 3.In our opinion the lower wear intensi

and the workpiece under test) to the short-time tensile yield strength of the softer contact body at the test temperature The value of t is simply a mea-sure of the resistance of the joint to shear The friction condition at the sur-face of a cutting tool will be similar to that for which the value of t was measured

The results of the wear resistance tests are given inFig 6.As can be seen, the wear resistance of HSS tools is 2.0–3.5 times lower than that of DCPM tools This reduction was associated with a significantly lower fric-tion parameter of DCPM compared to HSS (Fig 7), and a broadening of the range of normal friction (Fig 6) Within the normal friction range, the rate of wear for DCPM is much lower than that for HSS (Fig 6, curves 1–3) While the hardness and heat resistance values of the HSS T15 and DCPM are similar, the wear resistance of the latter is significantly higher (Table 3).In our opinion the lower wear intensity of the DCPM-tool mate-rial is related to the presence of titanium carbides in the structure and their subsequent transformation to oxygen-rich compounds during cutting When studied by secondary ion mass spectroscopy (SIMS), the analy-sis of typical wear craters revealed the formation of oxygen-containing phases The data inFig 8demonstrate that the transformation of titanium carbide into an oxygen-containing phase starts in the initial stage of wear (during the running-in process, Fig 8a) With further operation, there is increased surface oxide formation at the bottom of the wear crater This process is accompanied by stabilization of the wear processes (Fig 6 and 8b,c) and an expansion of the normal friction range Evidently, this is deter-mined by the phenomenon of self-organization that is connected with the emergence of secondary structures (titanium–oxygen compounds), which play the role of stable solid lubricants [25]

Figure 6 The dependence of the flank wear value of cutting tools on the cutting time: (1) HSS M2; (2) HSS T15; (3) DCPM Turning test data acquired with 1040 steel Parameters of cutting: speed (m=min): 55; depth (mm): 0.5; feed (mm=rev): 0.28

Figure 9 Microphotograph of tool friction surface with films of secondary structures: (a) general view of the surface using secondary electrons; (b) distribution

of oxygen close to the ‘‘built-up-crater’’ contact surface (SI, intensity of signals, arbitrary units)

electron spectroscopy, are given in Fig 9(b) In the left part of the micrograph (a), a build-up of 1040 C steel can be seen The right part of the micrograph shows the distribution of dispersed hardening phases in the HSS-based DCPM Angular (dark) particles of titanium carbide (less than 8 mm in cross-section) as well as dispersed tungsten and molybdenum carbides (less than 0.2–1.5 mm in diameter) are uniformly distributed in the HSS matrix In the surface layers of the tool material, we can observe

a zone of intense plastic deformation less than 5 mm in depth There, dis-persed particles of a titanium-containing phase have been drawn out parallel

to the wear surface, forming a discontinuous film The titanium carbides in the wear zone have been transformed into oxides (Fig 8and [23]) Titanium oxides are known to be much more plastic than titanium carbides, account-ing for the plastic deformation of the particles in the surface layers of the HSS-DCPM on cutting

These results are confirmed by Auger-spectroscopy Fig 9(b) repre-sents the distribution of the intensity of the characteristic Auger KLL lines for O, C, and the LMM (418 eV) line of Ti along the I–I direction in Fig 9(a) The analysis volume includes the built-up layer (of 1040 steel), the built-up layer=wear crater boundary, and the DCPM volumes beneath the wear crater At the interface, the titanium compounds show an increased concentration of oxygen and a decreased carbon content The observed change in chemical composition is related to the instability of titanium car-bide Due to the high cutting temperatures (in excess of 4508C) and pro-nounced affinity for oxygen, titanium adsorbs the latter from the environment and forms thin films of oxygen-containing compounds, in agreement with the SIMS data presented inFig 8 The total plastic defor-mation of these particles at the wear surface is greater than 600% The crys-tal structure of these compounds is believed to differ from the titanium oxides that would be obtained under equilibrium conditions (see below)

An understanding of the self-organizing phenomenon is critically important for the development of advanced tool materials A major interest

in these studies (from the point of view of materials science) is the nature of the secondary structures forming under severe cutting conditions According

to the principles of current tribology, one of the main methods to control friction is the creation of stable secondary structures at the tool surface The more stable are the secondary structures, the greater will be the tool life The development of protective secondary structures can be manipulated by alloying or by surface treatment technologies The type of secondary struc-tures formed during cutting depends strongly on the conditions of cutting and the type of the material under analysis

A detailed study of the physico-chemical parameters of the SSs formed during cutting using a tool made of DCPM was done using AES, ELS, and

EELFS methods To interpret the atomic structure of the tool wear surface, data obtained in this work by the EELFAS method were compared to TiC and TiO2standards Fig 10 presents the Fourier transform of data obtained

on analyzing the extended electron energy loss fine structure (EXELFS) for titanium carbide (TiC) with a cubic (B1) structure The positions of the main peaks (Fig 10a) are consistent with the interatomic distances for a (1 0 0) plane in the cubic lattice of titanium carbide (see Fig 10b)

Figure 10 (a) Fourier transform of EELFS close to the line of back-scattered electrons for TiC specimen, Ep¼1500 eV; (b) cubic lattice of titanium carbide (¼Ti;

¼ C)

Fig 11(a) shows data for TiO2with the rutile (C4) structure We can identify the type of bonds by using partial functions F(R) obtained from the analysis of the fine structure of spectra close to the characteristic Auger lines

of oxygen and titanium By comparing these data with those given in Fig 9(a), we can see that TiO2 has a more complex crystalline structure than TiC This explains the greater number of F(R)-function peaks The positions of the main peaks are again in good agreement with the

Figure 11 (a) Fourier transform of EELFS close to the line of back-scattered electrons for TiO2 specimen with rutile structure, Ep¼1500 eV; (b) cubic lattice of titanium oxide (¼Ti;
¼O)

interatomic distances for a (1 0 0) plane in the TiO2 lattice The complete analysis of all the peak positions by the Fourier transform method shows that the interatomic distances O–O and Ti–O in the secondary structures are different from those discussed in the literature (see Fig 11b) This may be related to a deviation from stoichiometry, or the interatomic dis-tances, measured from an analyzed volume that is only several angstroms thick near the surface, are different from the equilibrium values

The evolution of the atomic structure in the surface films on the wear crater of the cutting tool is well illustrated by the data given inFig 12.The oxygen-containing films in the wear crater are significantly enriched with titanium and oxygen after only 5 min of cutting (see Fig 12a–d) As this takes place, a periodicity in the arrangement of atoms of various types is observed both in the nearest coordination sphere and at greater interatomic distances, up to approximately 7 A˚ (see Fig 12b)

As noted above, the interatomic distances in these oxygen-containing films differ from those observed in equilibrium titanium oxides, including rutile (compare with Fig 11) The very thinnest films may be 2D (two-dimensional) phases whose atomic structure is close to the supersaturated a-solid solutions of oxygen in titanium After 15 min of cutting, the degree

of long-range order is reduced, while the intensities of peaks from higher order coordination spheres are less pronounced (see Fig 12c) After

30 min of cutting (Fig 12d), the translational symmetry at large interatomic distances disappears, and peaks at R> 4 A˚ are lost

The adaptability of the surface layer to external thermo-mechanical effects is the physical basis of such evolution The surface is gradually con-verted to an amorphous state during the wear process (after cutting times of about 15 min) When a steady-state condition is reached, i.e., after the devel-opment of the SS is completed, the surface generates amorphous-like films having an effective protective function The lattice instability of the solid solution of oxygen in titanium finally leads to complete amorphization of the water surface A similar effect was observed earlier from EELFS data for TiN-coatings on worn cutting tools [26]

Typical EELS spectra of TiC, TiN, TiO2obtained with a 30.0 eV pri-mary electron beam are shown inFig 13 The elastic peak has a 30 meV FWHM (full width at half maximum) The high-resolution structures of the spectra are represented at 1000 magnification after normalizing The experimental curves are approximated by Gaussian peaks in each spec-trum in the range of 1–9 eV energy loss

In the series of titanium compounds TiC–TiN–TiO, the number of 4s electrons in the atomic sphere of the metal decreases from the carbide to the oxide These electrons are transferred to the 2p orbital of the non-metal atom, and the band energy is lowered due to the increasing Ti–X attraction

this orbital is more completely filled in the case of oxygen than in either nitrogen or carbon For this reason, when the 3d Ti- and 2p X orbitals are hybridized, the contribution of the ds-electrons of Ti is less pronounced

in the oxide and more expressed in the nitride and carbide Consequently, the oxide has considerably less strength and hardness than the carbide or nitride [27]

The interaction of Ti–Ti atoms is realized at the expense of dp-elec-trons The metallic nature of this compound is related to the high density

of dp-electrons As seen inFig 13,the intensity of t1g ! t2gtransitions in the p-band is relatively insignificant in TiC, but it is much higher in TiO2 This implies that the density of conduction electrons is low in TiO2but it

is higher for TiC and TiN This is consistent with the electrical conductivity data of these compounds, which is extremely low for the dielectric TiO2, but 16,400 (Ohm m)1for TiC and TiN, respectively [27]

The replacement of carbon with oxygen in titanium compounds was shown to change their properties significantly Thus, the oxidation of TiC

at 823 K for 30 min influences the electronic structure of the material, the electron spectrum acquiring some features specific to TiO2 (see Figs 14a

Figure 14 Representative ELS spectra: (a) after oxidation of TiC by heating up to

823 K for 30 min in air; (b) wear surface of DCPM cutting tool after 5 min of operation; (c) wear surface of DCPM cutting tool after 30 min of operation The spectra were obtained using a 30.0 eV primary electron beam

and 13c) After oxidation of TiC, the intensity of the lines at 6.7 and 1.6 eV is substantially enhanced On oxidation, the titanium carbide loses its metallic properties and acquires those of a dielectric In this case, we observe a reduced concentration of conduction electrons and a localization of the elec-tron density both in the metal and non-metal atoms This is shown by the increased intensity of the peak at 1.6 eV corresponding to p-states in the 3dd-band of titanium (seeFig 13a)

It was noted earlier that an intense oxidation of TiC could be observed during the operation of a DCPM tool [20,22] In this case, the nature of the phase transformation differs significantly from that found on heating a TiC standard up to 823 K for 30 min Figure 14(b) and (c) presents electron energy loss spectra from the wear crater after 5 and 30 min of DCPM-tool operation As the wear time increases, the spectra display a somewhat increased intensity of peaks at 6.8 and 3.1 eV Peaks corresponding to plasmon losses (p1 and p2) appear, while the peak at 1.6 eV is significantly attenuated The thin SS films in the wear crater of the cutter are associated with the formation of supersaturated solid solution of oxygen in titanium due

to the oxidation of titanium carbide In this case, we observe an increase

in the electron density in the 2p orbital of the non-metal (peak 6.8 eV) as well as an enhanced filling of the ds-electron band of titanium atoms (peak 3.1 eV) These effects are similar to those encountered in the model oxida-tion of TiC (Fig 14) There are, however, substantial differences As the cut-ting time increases, the effects brought about by the crystalline structure

of phases become significantly weakened in the electron spectrum The splitting of the 3d orbital into p and s-states degenerates, the intensity of

t1g! t2g transitions is reduced as well as the density of p-electrons which are related to the long-range Ti–Ti bonds in the lattice (along the diagonals

in the (1 0 0) planes) These distinctive features of the electron structure are related to amorphization and to the increasing role of short-range inter-atomic bonds Of considerable interest is the appearance of plasmon loss peaks p1 and p2 in the spectra of Figs 14(b,c) due to the growing concen-tration of conduction electrons The delocalization of p-electrons close to the titanium atoms enhances the metallic nature of bonds in the amorphous films developed on the friction surface These specific traits of electronic and atomic structural change might help to explain the unique mechanical prop-erties in the secondary structures of the first type The high wear resistance and good frictional properties of DCPM tools are associated with complex structural and phase transformations on the surface, among them TiC oxi-dation and the development of thin protective amorphous films The SSs are saturated or supersaturated (amorphous) solid solutions of oxygen in titanium, whose electron structure is characterized by a high density of con-duction electrons giving metallic characteristics

These results show that the SSs formed during cutting not only increase the DCPM-tool life but also change friction characteristics as well The amorphous-like secondary structures of the first type behave like a solid lubricant with enhanced tribological properties [4]

Additional alloying of the DCPM might be beneficial For example, the partial substitution of titanium carbide by aluminum oxide, which is stable under cutting leads to a decrease in the friction coefficient (Fig 15)

Figure 15 Impact of the test temperature on the wear and friction characteristics

as determined from wear contact tests for the DCPM with 15% TiC þ5% Al2O3; 20% TiCþ2% BN and 20% TiCN additions

and in an increase in the wear resistance of the tool (Fig 16) The decrease

of the friction coefficient when Al2O3is added is important not only as it increases the wear resistance but also because it lowers the cutting tempera-ture at the tool surface [28,29] Alloying often cannot be implemented by the traditional metallurgical methods since this may induce an undesirable change in the properties of the cutting material We took a different approach by making small additions of low-density compounds, which are relatively unstable at the operational temperatures This allowed us to use this compound in relatively small quantities (up to 2 w%) with minimal possible impact on the bulk properties The solid lubricant (hexagonal BN) was chosen as the additional alloying compound [28] The high probability

of oxygen-containing secondary phases formed from BN during cutting was also taken into account The possibility that TiC and BN might oxidize and generate thin surface oxide films for exploitation in cutting tools can be assessed by a thermodynamic approach [27]

Secondary ion mass spectroscopy investigations have shown that on cutting DCPM with a boron nitride addition, oxygen-containing com-pounds develop at the wear-crater surface, associated with a set of parallel disassociation reactions of BC, BN, TiC leading to the formation of BO, TiO and TiB,N.Figure 17a–cpresents spectra of the positive and negative

Figure 16 Wear curves of friction contact materials: (1) DCPM with 20% TiC; (2) DCPM with 20% TiC and 2% BN; (3) DCPM with 15% TiC and 5% Al2O3; (4) DCPM with 20% TiCN Turning test data acquired with 1040 steel Parameters

of cutting: speed (m=min): 90; depth (mm): 0.5; feed (mm=rev): 028