Sintering of Ti ( C , N )-WC / Mo 2 C-( Ta , Nb ) CCo / Ni Cermets Investigated by CO and N 2 Outgassing

Cermets of the type Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni with changing [Mo]/([Mo] + [W]) ratio were subjected to an investigation of outgassing of CO and N2 upon sintering. Quantification of CO and N2 was performed by gas calibration, measurement of masses 12 (12C), 14 (14N) and 28 (28CO and N2), as well as C, N, O analysis of the samples before and after sintering. The formation of CO occurs at lower temperatures than that of N2, both gases being completely evolved already at solid-state sintering conditions. If pre-alloyed powders are employed in the starting formulation, the amount of evolved gases is substantially reduced, because part of the formation of mixed hard phases is anticipated. Changing binder composition from Co:Ni = 1:1 to 2:1 and 3:1 does not change the outgassing characteristics, while different batches of nominally the same Ti(C,N) powder can have significant influence. Mass spectrometry is a most valuable in situ tool for getting insight into the metallurgical reactions occurring upon sintering. These reactions result in the typical microstructure and influence the properties of cermets.


Introduction
Mass spectrometry (MS) was already used in the 1970s in hardmetal production with respect to characterisation of the sintering atmosphere [1,2].The latter influences the carbon balance substantially, which in turn has great influence on the quality of hardmetals.By use of flexible, handy and relatively inexpensive quadrupole mass spectrometers (QMS) [3], the method is readily applicable to laboratory and industrial sintering furnaces.Because the intensive interaction of gas phase with the powder compacts, this method was later adopted for in situ studies of the sintering phenomena occurring in hardmetals and cermets [4][5][6][7][8].
In our laboratory, already in 1990, we started to obtain deeper knowledge of the metallurgy of cermets by use of QMS [5,6] and noticed the considerable importance for optimising the sintering profile with respect to temperature and gas-phase pressure.As recently shown, MS investigations also supply important information to characterise Ti(C,N) powders originating from industrial processes [9].
Most of the equipment used for the investigation of such outgassing behaviour is thermo-analytical standard equipment with often a combination of DTA or dilatometry with MS.Such equipment covers substantial drawback, as the environment of the furnaces is not comparable to that of a sintering furnace (both, in production and research).Most furnaces of thermo-analysis are equipped with aluminium oxide ceramics, whereas sintering is performed by use of graphite heaters and trays.Hence, the furnace atmosphere differs substantially between MS investigation and sintering as in ceramic tubes oxygen diffusion takes place.Another drawback, especially for DTA-MS coupled thermo-analytical systems, is the small sample size and thus the great influence of the atmosphere on the sample.
To overcome these drawbacks and to perform investigations which are closer to the condition of cermet sintering, we modified a laboratory furnace.While an earlier version of such a furnace still contained silica tube [5,8], a construction which is not used in cermet sintering, the actual furnace with which the recent [9] and present investigations were made is a laboratory sintering furnace.

Starting Powders and Formulations
Powders from Treibacher Industrie AG, Austria, were used for establishing various formulations.Secondly, Ti(C,N)-WC/Mo 2 C-(Ta,Nb)C-Co/Ni cermet formulations of various alternatives were prepared, Table 1, Table 2, Table 3.They had constant Ti, Ta, Nb concentrations (in mol.%) and a constant binder-phase content of 10.8 vol.%, but a changing molar [Mo]/([Mo] + [W]) ratio, in turn with a constant total of [W] + [Mo].The carbon deficiency of Mo 2 C-containing formulations was compensated by the addition of carbon black.Details on the solid-state and microstructural properties of the whole series were presented in [10].The production batches of all these powders were identical throughout the study, but one nominally same powder had two batches, Ti(C 0.6 N 0.4 )-A and Ti(C 0.6 N 0.4 )-B.

QMS Analysis
In the furnace used for MS analysis, an inductively-heated crucible of molybdenum (70 mm diameter, 50 mm height) is centred in a recipient which is connected via an orifice to the recipient equipped with a PrismaPlus 200 quadrupole mass-spectrometer, QMS (Pfeiffer, Germany).The orifice separates the pressure in the sintering chamber (max.50 mbar) from the pressure in the analysis chamber (10 −6 mbar).A gas supply system (MKS, Germany) and a pressure regulation are attached to the furnace to provide constant pressure during analysis.
Calibration of the QMS for CO and N 2 was performed with defined continuous flow rates of He, N 2 and CO, at a constant total pressure in the sintering furnace of 7.9-8.0mbar.The latter was also established during analysis.The continuous flow rates of CO and N 2 were adjusted near the respective gas phase concentrations obtained during analysis of the cermets.The masses 12, 14 and 28 were measured to separate CO and N 2 , both having a mass of 28.Because of this, quantification the concentration of these phases in vol.% in the gas phase was possible.
The powder mixtures were processed in the same way such as for conventional sintering: drum milled in hardmetal-lined drums with hardmetal balls (WC-6 wt.% Co) for 24 h in cyclohexane, dried and pressed to cylinders without pressing aid.The pressed sample (12.5 mm diameter, 8 mm height) was placed on an Al 2 O 3 plate to avoid reaction with molybdenum.Heating was done linearly with a rate of 10 • C/min from 450-1500 • C.
The properties and microstructure of the samples was investigated by standard techniques described elsewhere [10].Analysis for C, N and O was performed by carrier-gas hot extraction.

Model Alloys Ti(C,N)-Co/Ni with Additions
Figure 1a-e shows CO and N 2 outgassing results of model mixtures of Ti(C 0.5 N 0.5 ) with Co:Ni = 1:1 binder and with additions of WC, Mo 2 C and (Ta,Nb)C.All outgassing curves show a small CO and N 2 peak (arrow A) at around 600 • C, which is representative of the release of these phases from the binder phase.In addition, after this peak and at around 700 • C, both the CO and N 2 concentrations start to increase (arrow B).
For the mixture Ti(C 0.5 N 0.5 )-Co/Ni, Figure 1a, this increase of CO cumulates to a distinct maximum at ca. 1030 • C (arrow C).Conversely, the N 2 concentration increases only slightly until 1250 • C. At 1250 • C, both the CO and N 2 signals show a sudden drop (arrow D).This drop is due to pore closure, which stops outgassing.Obviously, up to 1300 • C, Ti(C,N) releases only a very small amount of N 2 but a substantial amount of CO upon reaction with the binder phase.The oxygen content is almost entirely reduced at around 1300 • C, which is in correspondence with O analysis, which gives less than 0.1 wt.% O after analysis as compared to 0.5-0.6 wt.% O in the starting powder.Nitrogen is of course still present in the material, the evolution of which increases after the drop upon increasing temperature because of successive decomposition of the carbonitride.
If WC is admixed, Figure 1b, a distinct CO peak with a maximum at 840 • C appears (arrow A), which is due to the oxygen release of this compound.The CO outgassing characteristics of Ti(C 0.5 N 0.5 ) Metals 2019, 9, 427 4 of 13 also changes, as the CO signal has a shoulder at higher T. Substantial N 2 outgassing starts just above 1100 • C, reaching a maximum at 1270-1280 • C. Both the CO and N 2 signals reduce to background concentrations only at around 1420 • C (arrow B).The release of N 2 is due to the formation of a quaternary (Ti,W)(C,N) phase (see discussion).
Similar to the mixture containing WC, the mixture with Mo 2 C shows a CO maximum just above 800 • C (arrow A), representative of oxygen release of Mo 2 C (Figure 1c).Also, the CO outgassing characteristics of Ti(C,N) is influenced, and again a shoulder at the high T side is observed with a slowly decreasing CO evolution ending above 1400 Similar to the mixture containing WC, the mixture with Mo2C shows a CO maximum just above 800 °C (arrow A), representative of oxygen release of Mo2C (Figure 1c).Also, the CO outgassing characteristics of Ti(C,N) is influenced, and again a shoulder at the high T side is observed with a slowly decreasing CO evolution ending above 1400 °C.The N2 outgassing of Ti(C,N) increases at 1180 °C, influenced by the presence of Mo2C-again a consequence of quaternary phase formation (Ti,Mo)(C,N).For the mixture Ti(C 0.5 N 0.5 )-(Ta,Nb)C-Co/Ni (Figure 1d), there is no CO and N 2 evolution at around 800 • C.This is due to the much higher stability of oxygen in (Ta,Nb)C.Principal outgassing at higher temperatures is similar to the mixtures with Mo 2 C and WC, whereas the quantity of released N 2 is between the latter.At 1240 • C, two distinct local maxima for CO and N 2 can be observed.By comparison of Figure 1a with Figure 1b-d, it is evident-from slow reduction of gas evolution-that admixing of WC, Mo 2 C or (Ta,Nb)C results in a much slower pore closure than Ti(C,N)-Co/Ni.
Figure 1e shows a magnification of the CO and N 2 outgassing of the binder phase.They do not strictly start at the same temperature (arrows); N 2 evolution starts about 15 • C lower (see discussion).The CO outgassing (Figure 2a) of the various grades is quite similar which a small local maximum at 650 • C of the binder oxygen release, at 900 • C with the oxygen release of WC or (Ti,W)C, and a large peak starting just below 950 • C and ending at 1300-1350 • C. The latter is representative of the oxygen release of Ti(C,N).Interestingly, the CO formation is larger and the evolution maximum is shifted to lower temperatures the smaller the overall C/N ratio (the higher the nitrogen content) of the starting formulation.

Outgassing of Cermets
The N 2 outgassing (Figure 2b) of these grades shows that the largest amount of nitrogen is released from the mixture containing Ti(C 0.6 N 0.4 ) and not from that which contains the carbonitride with the highest nitrogen content Ti(C 0.5 N 0.5 ).The pre-alloyed formulation loses a very low amount of nitrogen.
Metals 2019, 9, x FOR PEER REVIEW 5 of 13 For the mixture Ti(C0.5N0.5)-(Ta,Nb)C-Co/Ni(Figure 1d), there is no CO and N2 evolution at around 800 °C.This is due to the much higher stability of oxygen in (Ta,Nb)C.Principal outgassing at higher temperatures is similar to the mixtures with Mo2C and WC, whereas the quantity of released N2 is between the latter.At 1240 °C, two distinct local maxima for CO and N2 can be observed.By comparison of Figure 1a with Figure 1b-d, it is evident-from slow reduction of gas evolution-that admixing of WC, Mo2C or (Ta,Nb)C results in a much slower pore closure than Ti(C,N)-Co/Ni.
Figure 1e shows a magnification of the CO and N2 outgassing of the binder phase.They do not strictly start at the same temperature (arrows); N2 evolution starts about 15 °C lower (see discussion).The CO outgassing (Figure 2a) of the various grades is quite similar which a small local maximum at 650 °C of the binder oxygen release, at 900 °C with the oxygen release of WC or (Ti,W)C, and a large peak starting just below 950 °C and ending at 1300-1350 °C.The latter is representative of the oxygen release of Ti(C,N).Interestingly, the CO formation is larger and the evolution maximum is shifted to lower temperatures the smaller the overall C/N ratio (the higher the nitrogen content) of the starting formulation.

Outgassing of Cermets
The N2 outgassing (Figure 2b) of these grades shows that the largest amount of nitrogen is released from the mixture containing Ti(C0.6N0.4) and not from that which contains the carbonitride with the highest nitrogen content Ti(C0.5N0.5).The pre-alloyed formulation loses a very low amount of nitrogen.For the non-pre-alloyed grade with Ti(C0.5N0.5),Ti(C0.6N0.4) and Ti(C0.7N0.3),there is a small peak at 780 °C (arrow A) which is due to Mo2C in the starting formulation (it increases if Mo2C is increased).It is absent for the grade containing Mo in pre-alloyed powder (Ti,Mo)(C,N) (yellow line).With an Mo2C addition, the main CO peak narrows when compared with the formulation without Mo2C  (compare with Figure 2a).Again, the CO evolution returns to the background level at 1400 °C, whereas that of the pre-alloyed grade (yellow line) has a shoulder on the high T side but is then released already at 1350 °C.
The nitrogen evolution of this Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni grade, shown in Figure 3b, starts at the same temperature (arrow A, T ≈ 1060 °C) as that of Ti(C,N)-WC-(Ta,Nb)C-Co/Ni (compare Figure 2b), but increases steeper so that the N2 evolution peaks are narrower.Interestingly, the mixture with Ti(C0.6N0.4)-Ashows again the largest N2 outgassing.The CO and N2 evolution of grades without WC are shown in Figure 4a,b.Again, grades with Mo2C powder and Ti(C0.5N0.5),Ti(C0.6N0.4)-A and Ti(C0.7N0.3),respectively, show a bump at around 780 °C (arrow A), as well as the pre-alloyed grade with Mo in form of (Ti,Mo)(C,N).The main CO peak is further narrowed as compared with WC and WC/Mo2C-containing grades, and again the prealloyed version has the smallest CO evolution with a high T shoulder.However, the CO evolution reduces steeply and ends at lower temperature (1260 °C), whereas the other grades show an evolution stop at 1330-1400 °C.(compare with Figure 2a).Again, the CO evolution returns to the background level at 1400 °C, whereas that of the pre-alloyed grade (yellow line) has a shoulder on the high T side but is then released already at 1350 °C.
The nitrogen evolution of this Ti(C,N)-WC/Mo2C-(Ta,Nb)C-Co/Ni grade, shown in Figure 3b, starts at the same temperature (arrow A, T ≈ 1060 °C) as that of Ti(C,N)-WC-(Ta,Nb)C-Co/Ni (compare Figure 2b), but increases steeper so that the N2 evolution peaks are narrower.Interestingly, the mixture with Ti(C0.6N0.4)-Ashows again the largest N2 outgassing.The CO and N2 evolution of grades without WC are shown in Figure 4a,b.Again, grades with Mo2C powder and Ti(C0.5N0.5),Ti(C0.6N0.4)-A and Ti(C0.7N0.3),respectively, show a bump at around 780 °C (arrow A), as well as the pre-alloyed grade with Mo in form of (Ti,Mo)(C,N).The main CO peak is further narrowed as compared with WC and WC/Mo2C-containing grades, and again the prealloyed version has the smallest CO evolution with a high T shoulder.However, the CO evolution reduces steeply and ends at lower temperature (1260 °C), whereas the other grades show an evolution stop at 1330-1400 °C.The N 2 evolution shown in Figure 4b is successively narrowed by increasing Mo 2 C amount (compare with Figures 2b and 3b) while the onset remains at the same temperature (arrow A) with the same sequence in peak maxima Ti(C 0.6 N 0.4 ) > Ti(C 0.5 N 0.5 ) > Ti(C 0.7 N 0.3 ), as for the other two cermet grades.The mixture with pre-alloyed (Ti,Mo)(C,N) ("50/50 PA") has a much lower N 2 evolution than that with Ti(C 0.5 N 0.5 ) ("50/50").

Variation of Binder-Phase Composition
The cermet grades specified in Table 1 were prepared with once again two different Co:Ni ratios, that is, Co:Ni = 2:1 and 3:1, keeping all other concentrations constant (see Table 2

Grades with WC and/or Mo 2 C in the Starting Formulation
For the Co:Ni = 2:1 and 3:1 grades an alternative Ti(C 0.6 N 0.4 ) powder, "Ti(C 0.6 N 0.4 )-B" was employed.This powder had a larger oxygen content than that used for the Co:Ni = 1:1 grade.Figure 5a,c,e contains the outgassing results of these grades for CO and Figure 5b,d,f for N 2 .
As can see seen in Figure 5 there is practically no difference, for both the CO (a), (c), (e) and N 2 (b), (d), (f) evolution, between the Co:Ni = 2:1 and 3:1 grades, for which the same Ti(C 0.6 N 0.4 ) starting powder "Ti(C 0.6 N 0.4 )-B" was used.A steep drop of CO outgassing occurs at around 1250 • C.However, for the formulations with Co:Ni = 1:1, the formation of CO is substantially lower.This formulation contains a different Ti(C 0.6 N 0.4 ) powder-"Ti(C 0.6 N 0.4 )-A"-with lower oxygen and higher nitrogen content (20 rel% less O and 2 rel% more N than Ti(C 0.6 N 0.4 )-B) but essentially the same FSSS (Fisher sub-sieve sizer) grain size of 1.55-1.65 µm.Hence, the differences between the grade with Co:Ni = 1:1 and that of 2:1 and 3:1 is not due to the different Co:Ni ratio.The different behaviour of formulations with different Ti(C 0.6 N 0.4 ) powders is also reflected in the nitrogen evolution, Figure 5b,d,f, and opposite to that of CO evolution.In this case, a much higher N 2 outgassing peak of the grade with Ti(C 0.6 N 0.4 )-A powder than that with the Ti(C 0.6 N 0.4 )-B powder is observed.
As with CO, again a steep drop in N 2 outgassing occurs at around 1250 • C in the grades with Ti(C 0.6 N 0.4 )-B).The significantly different outgassing behaviour of nominally similar powders is interesting.If microstructures of these grades are inspected (Figure 6a-i) a different grain size of the carbonitride phase intermediate in atomic weight (medium grey) can be observed.The use of Ti(C 0.6 N 0.4 )-A powder gives larger grains of this phase than the Ti(C 0.6 N 0.4 )-B powder.
Grades with (Ti,W)C and/or (Ti,Mo)(C,N) in the Starting Formulation Figure 7a,c,e shows the CO evolution of grades with three different binder phase compositions (Co:Ni = 1:1, 2:1 and 3:1) and for which the entire W and Mo content was added in form of "pre-alloyed" (Ti,W)C and/or (Ti,Mo)(C,N), Table 3.The CO evolution characteristics is quite similar among these grades.The evolved amount is lower than that for WC-and Mo 2 C-containing formulations.
The N 2 evolution curves show a very small outgassing of all pre-alloyed grades (Figure 7a-c).The evolution drops steeply at T = 1250-1300 • C for both gases and then increases again for nitrogen up to the maximum temperature of 1500 • C.

Variation of Binder-Phase Composition
The cermet grades specified in Table 1 were prepared with once again two different Co:Ni ratios, that is, Co:Ni = 2:1 and 3:1, keeping all other concentrations constant (see Table 2 2).

Discussion
The subsequent evolution of CO and N 2 reflects the strength of which carbon, oxygen and nitrogen is bonded in the respective phases.Carbon and oxygen for formation of CO stems either from an individual phase containing both elements or from the interaction of oxygen bonded in the lattice with carbon from the surrounding phases including free carbon.In the investigated temperature range, the weakest bonded oxygen is that of the Co/Ni binder phase.
The evolution of N 2 from the binder phase is interesting, as there is no significant amount of nitrogen in the binder phase which should give rise to a outgassing peak, such as seen in Figure 1a-e Metals 2019, 9, 427 9 of 13 for the model alloys, which have a higher amount of binder than the cermets.The only explanation is that nitrogen of the surrounding air is trapped in the pressed body, adsorbed to the particles' surfaces and released when CO is formed.Hence, both gases released by the binder phase appear at the same temperature.A closer look to this release shows (Figure 1e) that the onset of N 2 formation is slightly earlier than the onset of CO and the increasing slope is not as steep as for CO.The peak maxima differ by about 15 • C.This is proof of different bonding (lattice oxygen vs. desorption of nitrogen).The next CO evolution higher in temperature stems from Mo2C and WC, which evolve CO practically at the same temperature because of quite similar stability of their oxides.The smaller CO peak is due to a smaller concentration of WC (Figure 1b) than of Mo2C (Figure 1c).At temperatures around 1050 °C, the maximum of CO released can be found from the interaction of Ti(C,N) with the binder phase.The onset of this peak and the increasing slope is quite similar, whereas the hightemperature side and decreasing behaviour of this CO evolution changes.It is influenced by the successive formation of a hard-phase alloy such as (Ti,W)(C,N) and (Ti,Mo)(C,N): compare Figure 1a with Figure 1b,c  The next CO evolution higher in temperature stems from Mo 2 C and WC, which evolve CO practically at the same temperature because of quite similar stability of their oxides.The smaller CO peak is due to a smaller concentration of WC (Figure 1b) than of Mo 2 C (Figure 1c).At temperatures around 1050 • C, the maximum of CO released can be found from the interaction of Ti(C,N) with the binder phase.The onset of this peak and the increasing slope is quite similar, whereas the high-temperature side and decreasing behaviour of this CO evolution changes.It is influenced by the successive formation of a hard-phase alloy such as (Ti,W)(C,N) and (Ti,Mo)(C,N): compare Figure 1a with Figure 1b,c.Another sign of the successive formation of mixed hard phases can be observed from   3.
Comparing the Ti(C,N) powders with different C/N ratio there is no significant different in peak shape and onset.Form this it follows that the dissolution of Ti(C,N) appears at the same temperature and is not dependent on the C/N ratio of these ternary powders.
When CO and N2 evolution peaks corresponding to Ti(C,N) are compared for the different  3.
Comparing the Ti(C,N) powders with different C/N ratio there is no significant different in peak shape and onset.Form this it follows that the dissolution of Ti(C,N) appears at the same temperature and is not dependent on the C/N ratio of these ternary powders.
which have the same Ti(C,N) and also the outgassing characteristics for all Co:Ni binders and with same Ti(C,N) powder in Figure 7.
However, the outgassing characteristics change substantially if only the type of Ti(C 0.6 N 0.4 ) powder changes.This is also seen in the microstructure; compare the microstructures in left column of Figure 6a,d,g with the middle and right columns.It gives proof of the interdependency of outgassing characteristics with microstructural features.

Conclusions
There is an increasing interest in Ti(C,N)-based cermets for various cutting applications, both metal and non-metal type of materials, as well as applications in electronic industry [12].This is due to the practically inexhaustible variability of formulations together with the broad variety of available hard-phase powders in different alloy status and grain sizes.In addition, the raw material supply risk for cermets is low.
However, the formation of a pore-free microstructure with complicated hard-grain structures is a complex process, to be studied by various techniques also upon interruption of sintering cycles [13] to "freeze" the high-temperature state if an in situ method is unavailable, such as with light-optical or SEM metallography or electron-probe microanalysis [14].Outgassing of CO and N 2 is always accompanied with sintering and also a necessary precondition for forming the typical cermet grains.At low nitrogen activity nitrogen, outgassing is also necessary for inner rim formation and starts in the solid state continuing in the eutectic liquid.If the nitrogen activity in the gas phase is increased outgassing cannot occur and the typical core-rim-type structure does not form.
Mass spectrometry is also important for getting immediate response if the raw powders are changed [9] or the material composition is varied, for example if carbonitrides from different sources are employed, or if the pre-alloyed status of hard-phase powders or the binder phase composition is modified.For such a situation, MS supplies valuable information for the re-design and optimisation of the sintering profile and the sintered materials.Since our technique is quantitative with a precision of about ±10 rel%, the amount of outgassed CO and N 2 in the various grades can be directly compared, also for a given temperature range.This was proven by comparison of total outgassed CO and N 2 with difference in C and N concentration before and after analysis (carrier-gas hot extraction), see also [8].
In the present study we have investigated cermet grades, which were already systematically studied for with respect to microstructure, hardness fracture toughness as well as liquid-phase formation temperature (DTA) and mass change (TG) in previous investigations [10,13], for a further step in completion of the knowledge of these materials.The sample size of the present study was comparable to that of an indexable insert (≈0.5-1 cm 3 ), much larger than that used in usual DTA-MS analytical equipment (≈ mm 3 ).

3. 2 . 1 .
Figure 2a,b shows the outgassing of Ti(C,N)-WC-(Ta,Nb)C-Co/Ni cermets with different ratios of the Ti(C,N) powder (i.e., Ti(C 0.5 N 0.5 ), Ti(C 0.6 N 0.4 ) and Ti(C 0.7 N 0.3 )), as well as one grade for which the entire W content was admixed in form of a pre-alloyed powder (Ti,W)C.The CO outgassing (Figure2a) of the various grades is quite similar which a small local maximum at 650 • C of the binder oxygen release, at 900 • C with the oxygen release of WC or (Ti,W)C, and a large peak starting just below 950 • C and ending at 1300-1350 • C. The latter is representative of the oxygen release of Ti(C,N).Interestingly, the CO formation is larger and the evolution maximum is shifted to lower temperatures the smaller the overall C/N ratio (the higher the nitrogen content) of the starting formulation.The N 2 outgassing (Figure2b) of these grades shows that the largest amount of nitrogen is released from the mixture containing Ti(C 0.6 N 0.4 ) and not from that which contains the carbonitride with the highest nitrogen content Ti(C 0.5 N 0.5 ).The pre-alloyed formulation loses a very low amount of nitrogen.

3. 2 . 1 .
Figure 2a,b shows the outgassing of Ti(C,N)-WC-(Ta,Nb)C-Co/Ni cermets with different ratios of the Ti(C,N) powder (i.e., Ti(C0.5N0.5),Ti(C0.6N0.4) and Ti(C0.7N0.3)), as well as one grade for which the entire W content was admixed in form of a pre-alloyed powder (Ti,W)C.The CO outgassing (Figure2a) of the various grades is quite similar which a small local maximum at 650 °C of the binder oxygen release, at 900 °C with the oxygen release of WC or (Ti,W)C, and a large peak starting just below 950 °C and ending at 1300-1350 °C.The latter is representative of the oxygen release of Ti(C,N).Interestingly, the CO formation is larger and the evolution maximum is shifted to lower temperatures the smaller the overall C/N ratio (the higher the nitrogen content) of the starting formulation.The N2 outgassing (Figure2b) of these grades shows that the largest amount of nitrogen is released from the mixture containing Ti(C0.6N0.4) and not from that which contains the carbonitride with the highest nitrogen content Ti(C0.5N0.5).The pre-alloyed formulation loses a very low amount of nitrogen.

3. 2 . 2 .
Ti(C,N)-WC/Mo 2 C-(Ta,Nb)C-Co/Ni and Ti(C,N)-(Ti,W)C/(Ti,Mo)(C,N)-(Ta,Nb)C-Co/Ni, Co:Ni = 1:1 If a part of the WC is replaced by Mo 2 C ([Mo]/([Mo] + [W]) = 0.60) the local CO evolution maxima of binder and CO oxygen of WC/Mo 2 C remain essentially unchanged at 620 • C and 880 • C (Figure 3a).For the non-pre-alloyed grade with Ti(C 0.5 N 0.5 ), Ti(C 0.6 N 0.4 ) and Ti(C 0.7 N 0.3 ), there is a small peak at 780 • C (arrow A) which is due to Mo 2 C in the starting formulation (it increases if Mo 2 C is increased).It is absent for the grade containing Mo in pre-alloyed powder (Ti,Mo)(C,N) (yellow line).With an Mo 2 C addition, the main CO peak narrows when compared with the formulation without Mo 2 C Metals 2019, 9, 427 6 of 13(compare with Figure2a).Again, the CO evolution returns to the background level at 1400 • C, whereas that of the pre-alloyed grade (yellow line) has a shoulder on the high T side but is then released already at 1350 • C.The nitrogen evolution of this Ti(C,N)-WC/Mo 2 C-(Ta,Nb)C-Co/Ni grade, shown in Figure3b, starts at the same temperature (arrow A, T ≈ 1060 • C) as that of Ti(C,N)-WC-(Ta,Nb)C-Co/Ni (compare Figure2b), but increases steeper so that the N 2 evolution peaks are narrower.Interestingly, the mixture with Ti(C 0.6 N 0.4 )-A shows again the largest N 2 outgassing.Metals 2019, 9, x FOR PEER REVIEW 6 of 13

3. 2 . 3 .
Ti(C,N)-Mo 2 C-(Ta,Nb)C-Co/Ni and Ti(C,N)-(Ti,Mo)(C,N)-(Ta,Nb)C-Co/Ni, Co:Ni = 1:1 The CO and N 2 evolution of grades without WC are shown in Figure 4a,b.Again, grades with Mo 2 C powder and Ti(C 0.5 N 0.5 ), Ti(C 0.6 N 0.4 )-A and Ti(C 0.7 N 0.3 ), respectively, show a bump at around 780 • C (arrow A), as well as the pre-alloyed grade with Mo in form of (Ti,Mo)(C,N).The main CO peak is further narrowed as compared with WC and WC/Mo 2 C-containing grades, and again the pre-alloyed version has the smallest CO evolution with a high T shoulder.However, the CO evolution reduces steeply and ends at lower temperature (1260 • C), whereas the other grades show an evolution stop at 1330-1400 • C. Metals 2019, 9, x FOR PEER REVIEW 6 of 13
).The intermediate grade, which contained both W and Mo, had a molar ratio of [Mo]/([Mo] + [W]) = 0.5 for Co:Ni = 2:1 and 3:1 and [Mo]/([Mo] + [W]) = 0.6 for Co:Ni = 1:1.This difference is due to a reduction of sample number (to prepare only one intermediate grade with Mo and W, instead of 4), and has no visible impact on the outgassing behaviour and on the microstructure [10].
. Another sign of the successive formation of mixed hard phases can be observed from a comparison of the outgassing characteristics of mixtures with Ti(C,N) + WC/Mo2C with alloys with pre-alloyed phases (Ti,W)(C,N) and (Ti,Mo)(C,N).Upon using the latter, a part of the reaction which forms quaternary phases during sintering is no more present and a much lower outgassing quantity is observed (Figures 2a,b, 3a,b and 4a,b).The most striking difference of formulations with
the outgassing characteristics of mixtures with Ti(C,N) + WC/Mo 2 C with alloys with pre-alloyed phases (Ti,W)(C,N) and (Ti,Mo)(C,N).Upon using the latter, a part of the reaction which forms quaternary phases during sintering is no more present and a much lower outgassing quantity is observed (Figure2a,b, Figure 3a,b and Figure 4a,b).The most striking difference of formulations with Ti(C,N) + WC/Mo 2 C and pre-alloyed (Ti,W)C and (Ti,Mo)(C,N) powders can be seen from the nitrogen evolution behaviour, Figures 2b, 3b and 4b.The release of nitrogen of these powder mixtures is generally much lower and appears at higher temperatures than that of mixtures with Ti(C,N) and WC or Mo 2 C. Metals 2019, 9, x FOR PEER REVIEW 10 of 13