The Effect Of Chemical Composition On Microstructure And Mechanical Properties In Ti(C,N)-based Cermets

In this investigation,Ti(C,N)based cermets were prepared by powder metallurgy technique.The effects of C,WC,Mo and Carbon nanotubes(CNTs)addition on the microstructure and mechanical properties of Ti(C,N)-based cermets were investigated by X-ray difraction(XRD),scanning electronmicroscopy(SEM),energy dispersive X -ray analysis(EDS).The microstructure,properties and the development process of Ti(C,N)-based cermets were outlined,and the development trends were pointed out;meanwhile,the development trend of carbon nanotubes(CNTs)reinforce composites was also briefed.Based on above work,the purpose and significance of this dissertation were pointed out.The raw materials,manufacturing process and testing methods were briefly introduced.Ti(C,N)-Co cermets were prepared with TiC and TiN as raw materials,Co as a binder,and the effects of C,WC and Mo addition on the microstructure and mechanical properties of Ti(C,N)based cermets were studied.Experimental results show that the microstructures of Ti(C,N)based cermets with C,WC and Mo addition all exist black core-grey rim structure and white core-grey rim structure,and of which most are typical black core.The amount of white core-grey rim structure and the carbonitride grain size increase and the volume fraction of undissolved TiC cores decreases with the increase of carbon content;the transverse rupture strength and the hardness exhibit a similar trend,i.e.first increases with the increase of carbon content,reaches the first peak value,then decreases with further increase of carbon content,reaches the lowest value at 1.5wt%C, subsequently reaches the highest value at 2.0wt%C,and decreases again with further increase of carbon content,while the fracture toughness of the cermets does not increase monotonously with the increase of carbon content.The microstructure has a trend to become finer with the increase of WC content;the transverse rupture strength and the hardness increase with the increase of WC content, the fracture toughness decreases with increasing WC content,but the change is not obvious when WC addition goes from 10 to 20wt%.Because of the formation of a Mo-rich rim and the improvement of the wettability between ceramic phase and metallic phase,the microstructure has an obvious trend to become finer with the increase of molybdenum content,when the added Mo exceeds 10wt%,ultra-fine TiC-based cermet with an average particle size of less than 0.5μm is obtained;the transverse rupture strength monotonously increases with the increase of Mo content, the hardness and the fracture toughness firstly increase and then decrease with increasing Mo content, and the maximum values of the hardness and the fracture toughness are found with 10wt%Mo and 5wt%Mo addition,respectively.Carbon nanotubes(CNTs)with tubular structures and excellent mechanical properties offer a kind of nanosized reinforcement for composite materials.So CNTs were incorporated into the matrix of Ti(C,N)cermets,CNTs were deposited with Co by electroless plating and the Ti(C,N)-Co cermets with various CNTs mass fraction were made by vacuum sintering.The effect of different CNTs contents both deposited and undeposited Co on the microstructure and mechanical properties of the cermets was studied.Results show that the microstructures of cermets exist black core-grey rim structure and white core-grey rim structure in Ti(C,N)-Co cermets with CNTs both deposited and undeposited Co,the fracture toughness and the hardness are enhanced with the addition of a very small amount of CNTs,then decreased with further increasing the content of CNTs.The fracture toughness of cermet with 0.5wt%CNTs deposited Co is the highest.The hardness of cermet with 1wt%CNTs deposited Co is the highest.They are about 112.8%and 43.7%higher than that of the cermet without CNTs addition,respectively.The fracture toughness of cermet with 2.5wt%CNTs undeposited Co are the highest.Its fracture toughness is about 87.7%higher than that of the cermet without CNTs addition.The hardness of cermet with 1wt%CNTs undeposited Co is the highest.Its hardness is about 58.6%higher than that of the cermet without CNTs addition consolidated under the same process.It shows that CNTs both deposited and undeposited Co can help to improve the fracture toughness and the hardness of Ti(C,N)-Co cermet.The effect of different CNTs contents both deposited and undeposited Ni on the microstructure and mechanical properties of Ti(C,N)-Ni cermets was studied.Experimental results show that the diffraction peaks of binder Ni move to the right with the increase of CNTs addition;this phenomenon is not visible until the CNTs addition reach to 2.5wt%,the diffraction peaks move to the right so visiblely that the third peak almost lap over the diffraction peaks of Ti(C,N).There were classical dark core-grayish rim structure and bright core-grayish rim structure in the microstructure of cermets.The volume fraction of bright core-grayish rim increases with increasing CNTs addition, so dose the volume fraction of pore.With the increase of CNTs deposited Ni,the amount of pores increases,the relative density decreases,and the extent of increasing or decreasing is a little until the CNTs addition reaches to 5.0wt%.Those are relative with the agglomerating of CNTs.The changing direction of hardness is basically accordant with that of relative density.The fracture toughness increases firstly,and then decreases,and the peak value present at 2.5wt%(15.94MPa·m^1/2).It is about 55.9%higher than that of the cermet without CNTs addition(10.19 MPa·m^1/2).With the increase of CNTs undeposited Ni,The fracture toughness and the hardness increases firstly,and then decreases.The fracture toughness and the hardness of cermet with 1wt%CNTs deposited Ni are 92.0 HRA and 16.6MPa·m^1/2.They are about 0.2%and 62.7%higher than that of the cermet without CNTs addition,respectively.It can be drawn that CNTs both deposited and undeposited Ni can play an active role in improving fracture toughness of Ti(C,N)-Ni cermets.Toughening mechanisms of CNTs reinforced Ti(C,N)-based cermets were studied.The main toughening mechanisms are pulling-out of CNTs,bridging of CNTs and residual stress toughen -ing,crack bowing,crack deflection and crack branching toughening.It is suggested that the increment in the mechanical properties of CNTs reinforced Ti(C,N)-based cermets is due to the combined effects of toughening mechanisms.Finally,the whole works were summarized.