|Title of Invention
SURFACE AND SUBSURFACE MODIFICATION OF THE HARDMETAL TO GET WEAR RESISTANT SURFACE LAYERS AND TOUGH INTERMEDIATE LAYERS IN THE SINTERING PROCESS
|Disclosed herein is a method for the formation of wear resistant layers in combination with tough sub-surface layers on hard metals comprising the step of sintering of the hardmetal varying pressures from 0.02 mbar to 100 mbar, different active gases like methane, hydrogen, carbon monoxide, argon and nitrogen in combination with vacuum (0.003 mbar to 0.5 mbar) at different temperature ranges from 500 degree C to 1600 degree C to obtain the ceramic layers of thickness from 1 micron upwards on top of the hardmetal and the said active gases being used in different proportions combined with vacuum during the heating and cooiing cycles to obtain the tough intermediate layers of thickness 1 micron to 40 micron.
|The invention is about the process for the formation of wear resistant layers in combination with tough sub-surface layers on hardmetals in the process of sintering.
PRESENT STATE OF THE ART:
The wear resistant surface layers are composed of ceramic carbides/carbo-nitrides/nitrides/borides either present as individual layers or as a combination of different layers with a metallic bindery varying from 0% to 30% mass and the sub-surface layers are free of cubic carbides, nitrides and borides with a metallic binder content varying from 5 to 30% mass. The hardmetal subsfrate contains 95% to 75% wear resistant matoial with the binder made of cobalt or/and nickel or/and molybdenum or/and aluminium varying from 5 to 25% mass.
As per known technology the wear resistant ceramic surface on hardmetal parts with an intermediate tough layer are obtained primarily in two stages.
Sintering of hardmetal parts with the tough layer on the surface of the sintered hardmetal.
Gradient smtering of hardmetals in which green compacts are subjected to heat-treatment (sintering) either under vacuum or reducing/inert atmosphere to obtain the sintered hardmetal. The tough layer is obtained on the siuface using a suitable process cycle which dissociates the cubic carbides, nitrides and borides present on the surface and creating a surface layer free of cubic compounds.
Subsequently, coating processes like CVD, PVD, PACVD, MTCVD, etc are employed where in the sintered hardmetal is subjected to deposition of different ceramic compounds on the surface of the hardmetal either through a chemical process or ionic process. A coated hardmetal is produced with a wear-resistant coating produced in a separate coating process and the integrated intermediate tough layer and hardmetal substrate produced in the process of gradient sintering.
In the known method the cost is high due to a separate post sintering coating operation.
No metallurgical bond between the ceramic coating and the hardmetat substrate leading to limitations in coating adhesion.
The coatings are always pure ceramic layers with no options for the formation of multiphased ceramic layers with a binder resulting in brittle coatings.
Coated hardraetal cannot be reworked to obtain the tough intamediate sub-surface layer.
OBJECT OF THE INVENTION:
The object of the present invention is to get a ceramic coating on hardmetal at low cost, with a better bond between the coating and the substrate and to reduce the brittleness of the coated layers by producing the coating in the sintering process.
It is a functionally gradient material wherein the surface layers are wear resistant followed by a tough sub-surface layer, which is tougher than the remaining portion of the hardmetal tool.
Functionally in a metal cutting application, the surface of the tool needs to be more wear resistant to improve die cutting process. But in a roughing application wherein the tool is subjected to shock loads, cracks emanate from the hard wear resistant surface layer, which should be impeded from proceeding to the core of the tool. A very tough layer sub-surface layer has the ability to arrest the crack propagation. In the surface hardened hardmetal as mentioned in co-pending application no.l605/MAS/96, formation of alternate layers of wear resistance and toughness is not possible.
In the conventional tool, tough layers are created on the surface during sintering and thro a stand alone process called the CVD or Chemical Vapour Deposition ceramic coatings are deposited on HM tools to improve the surface hardness or wear resistance. But in the new process the combined selective hardening & toughening is carried out in one single operation of the sintering process.
Since the basic hardmetal itself is tailored to get the surface layers of high wear resistance & sub-surface of high toughness, the bond between the layers and the tools is strong.
In CVD, the layers deposited are external and the bonding strength with parent hardmetal has its limitation.
During the normal sintering cycle at varying pressures from 0.02 mbar to 100 mbar different active gases like methane, hydrogen, carbon monoxide, argon and nitrogen are used in combination with vacuum (0.003 mbar to 0.5 mbar) at different temperature ranges from 500 degree C to 1600 degree C to obtain the ceramic layers of thickness from 1 micron upwards on top of the hardmetal. In the same sintering cycle these active gases are used in different proportions combined with vacuum during the heating and cooling cycles to obtain the tough intermediate layers of thickness 1 micron to 40 micron. The flow rates of these gases vary from 1 It/rain to 60 Its/min. The hard metal m addition to tungsten carbide and the binder phase contains alloying carbides and nitrides not less than 1%.
Hardmetals contain a uniform mixture of varying levels of tungsten carbide (WC), hard & wear resistant ceramic carbides and nitrides of the elements of Group:IV B and V B of the periodic table like titanium, tantalum, zirconium etc., along with the soft elements of the VIII like cobalt, nickel etc.
In the powder metallurgy of Hardmetals, the powder mixtures containing all these compounds & elements are compacted known as the green compacts. Green compacts are soft and contain high level of porosity which is removed in the subsequent process
called sintering. Sintering consolidates the green compacts removing all the pores and makes the hardmetals hard. It also results in uniform distribution of all the phases containing the carbides and nitrides and the soft metals. This results in isotropic characteristics.
There is also a known special process of making hard the carbides and nitrides of the elements of Group: IV B and V B migrate to the core for creation of a tough layer on the surface.
These surface layers are formed over varied compositions of hardmetals i.e hardmetals containing different contents of Co. Ni, Ti, etc. as specified herein. Since the hardlayers are primarily formed by the migration of Ti/Ta to the surface and the tough layers are formed by the inward migration of Ti/Ta, different ratios of gases are used for different compositions and even for the same composition the ratios vary depending on how much of Ti or Ta reqiiire to be migrated.
1. A method for the fonnation of wear resistant layers in combination with tough sub-surface layers on hard metals comprising the step of sintering of the hardmetal at a pressure from 0.02 mbar to 100 mbar, different .active gases like methane, hydrogen, carbon monoxide, argon and nitrogen in combination with vacuum (0.003 mbar to 0.5 mbar) at a temperature range from 500 degree C to 1600 degree C to obtain the ceramic layers of thickness from 1 micron upwards on top of the hardmetal, and the said active gases being used in combination with vacuum during the heating and cooling cycles to obtain the tough intermediate layers of thickness 1 micron to 40 micron.
|Indian Patent Application Number
|PG Journal Number
|Date of Filing
|Name of Patentee
|M/S. WIDIA (INDIA) LIMITED
|NO.8/9TH MILE, TUMKUR ROAD, POST BAG 7300, BANGALORE - 560 073
|PCT International Classification Number
|PCT International Application Number
|PCT International Filing date