Patent Application
20160002515
The present disclosure relates to coated superabrasive abrasive particles and its method of manufacturing them, more specifically, to the glass coated cubic boron nitride (cBN) particles and the method of making them.
Vitreous bond (vit-bond) grinding wheels made with cubic boron nitride (cBN) superabrasive materials are commonly used for grinding applications. Due to the nature of the CBN having hardness next to diamond, the grinding wheel made with CBN abrasives can be applied to grind ferrous and non-ferrous steels and super-alloys and possesses low wheel wear, high grinding ratio and good surface finish. CBN abrasives make it possible in grinding industry to shorten the grinding cycle time and improve the productivity through an accelerated grinding. However, the surface of the work piece ground may be caused with a higher roughness if it is ground at accelerated grinding condition which likely makes CBN abrasives pull out. Therefore, enhancement of the crystal retention in the grinding wheel bond is critical to the performance.
As a result, it can be seen that there is a need for a grinding tool made from superabrasive composite material to be used in a tough demanding operation, such as accelerated grinding condition.
In one embodiment, a method of making coated superabrasive material may comprise steps of blending superabrasive particles with a binder phase; mixing glass frits into the blended mixture at a first predetermined temperature; sieving out the superabrasive particles which were encapsulated with the glass frits; and mixing a spacer with the encapsulated superabrasive particles and heating the spacer with the encapsulated superabrasive particles up to a second predetermined temperature.
In another embodiment, a coated superabrasive material may comprise a core comprising a superabrasive crystal; and a glass coating evenly covering at an outside surface of the core, wherein the glass coating ranges from about 1 wt % to about 15 wt % of the superabrasive crystal and has a thickness from about 1 micron to about 2 microns.
In yet another embodiment, a method of making coated superabrasive material may comprise steps of blending glass frits with a plurality of wetted superabrasive particles; heating the blended mixture to a first predetermined temperature; and mixing a spacer with the encapsulated superabrasive particles and heating the spacer with the encapsulated superabrasive particles up to a second predetermined temperature.
The foregoing summary, as well as the following detailed description of the embodiments, will be better understood when read in conjunction with the appended drawings. It should be understood that the embodiments depicted are not limited to the precise arrangements and instrumentalities shown.
An embodiment may provide superabrasive particles with coating outside the superabrasive particles. The coated structure may possess a high grinding ratio while maintaining a competitive grinding power consumption during vitreous-bond steel grinding. The coated superabrasive particles, such as cBN or diamond particles, for example, may have a core and a coating outside the core. The superabrasive particles may be grown under high pressure and high temperature.
Cubic boron nitride (cBN) particles are known to be produced from hexagonal boron nitride catalyst systems, such as alkali and alkaline earth metal nitrides, under high pressure and high temperature for a time period sufficient to form the cubic structure. The reaction mass is maintained under pressure and temperature conditions that thermodynamically favor the formation of cubic boron nitride crystal. The cubic boron nitride is then recovered from the reaction mass using a combination of water, acidic solutions or caustic chemicals using recovery methods known in the art. It should be noted that other methods of producing cubic boron nitride are known, i.e., cubic boron nitride prepared via a temperature gradient method or a shock wave method, and modification of the process taught in the instant application may be used to produce the abrasive grains having unique features.
Any combination of starting ingredients, which provide both the hexagonal boron nitride and the catalyst may be employed. An embodiment of the starting reaction mixture may contain a source of boron, a source of nitrogen, and a source of catalyst metal. The source of the boron may be elemental boron, hexagonal boron nitride, or material such as one of the boron hydrides which may decompose to elemental boron under conditions of the reaction. The source of nitrogen may be either hexagonal boron nitride, or a nitrogen-containing compound of a catalyst metal which may provide a source of nitrogen under reaction conditions. The catalyst metal may be employed as the elemental metal or a catalyst compound which may decompose to the catalyst metal or to the catalyst metal nitride under reaction conditions.
The process may be carried out in any type of apparatus capable of producing the pressures and temperatures used to manufacture the superabrasive. An apparatus that may be used is described in U.S. Pat. Nos. 2,941,241 and 2,941,248. Examples of other apparatus include belt presses, cubic presses and split-sphere presses.
The apparatus includes a reaction volume in which controllable temperatures and pressures are provided and maintained for desired periods of time. The apparatus disclosed in the aforementioned patents is a high pressure device for insertion between the platens of a hydraulic press. The high pressure device consists of an annular member defining a substantially cylindrical reaction area, and two conical, piston-type members or punches designed to fit into the substantially cylindrical reaction area, and two conical, piston-type members or punches designed to fit into the substantially cylindrical portion of the annular member from either side of the annular member. A reaction vessel which fits into the annular member may be compressed by the two piston members or six piston members to reach the desired pressures in the manufacturing the grains having unique features. The temperature necessary is obtained by a suitable means, such as, by induction heating, direct or indirect resistive heating or other methods.
As shown in
The superabrasive crystal may have smooth surface in one embodiment, In another embodiment, the superabrasive crystal may have uneven surfaces 16. The uneven surface 16 may have spikes 19 or pits 16 on the surfaces. The superabrasive crystal may have toughness index more than 85. Superabrasive material, such as cubic boron nitride (cBN), is often used in grinding hard ferrous alloy work pieces due to cBN's relatively non-reactivity with ferrous work pieces. Accordingly, cBN materials often are formed into grinding and machining tools. The toughness of the cBN crystals, as measured by a standard friability test, may be a factor in grinding performance. The friability test involves ball milling a quantity of product under controlled conditions and sieving the residue to measure the breakdown of the product. The toughness index (TI) is measured at room temperature. The thermal toughness index (TTI) is measured after the product has been fired at a high temperature. In many cases the tougher the crystal, the longer the life of the crystal in a grinding or machining tool and, therefore, the longer the life of the tool.
The core 12 may have a single crystal structure in one embodiment. In another embodiment, the core 12 may have multiple crystal structures. The single crystal structure or multi crystal structures of the core 12 may be substantially faceted. The term “facet”, as used herein, refers to flat face on geometric shapes.
As shown in
Various methods of coating may be used for glass coating, such as chemical vapor deposition (CVD), physical vapor deposition (PVD), sol-gel or spray dry coating, or solid reaction coating, for example.
One or more steps may be inserted in between or substituted for each of the foregoing steps 32-38 without departing from the scope of this disclosure.
According to another embodiment, a method of making coated superabrasive material may comprise steps of blending glass frits with a plurality of wetted superabrasive particles; heating the blended mixture for a first predetermined temperature, ranging from about 55° C. to about 90° C., for example, sieving out the superabrasive particles which were encapsulated with glass frits; and mixing a spacer into the mixed glass frits and heating up for a second predetermined temperature, ranging from about 600° C. to about 1100° C., for example. The method may further comprise sieving out glass coated superabrasive particles from the spacer since the spacer may have a particle size smaller than the superabrasive particles. The spacer may be selected from a group consisting of a metal, a metal alloy, or a metal oxide. In one embodiment, the spacer may be aluminum oxide.
The superabrasive particles may be selected from a group consisting of cubic boron nitride, diamond and diamond composite materials. The superabrasive particles may be mixed with a binder phase, such as water, ethylene glycol, sugar, polyethylene glycol, glue, to form wetted superabrasive particles. The mixture of the superabrasive particles with the binder phase may be dried out before mixing with the spacer.
About 10 g of CBN 1000 80/100 mesh abrasives were weighed and poured into a 100 ml beaker where about 5 ml Glycol solution was prepared. The mixture was sonicated for about 2 mins followed by stirring sufficiently with a stick to make sure that the CBN was wet by Glycol completely. The beaker was tilted and Glycol solution was poured from the beaker until no obvious Glycol was left. Ferro glass frits (90741F-softening temperature of 620° C. and about 10-20 micron particle size range) 10 grams were weighed and split into two equal portions. The first portion (5 grams) was poured into a plastic bottle (about 200 ml sized bottle) to cover the entire bottom of the bottle. Subsequently, Glycol wetted CBN crystals were spread into the bottle evenly. The mixture was slightly stirred for 3 seconds. The mixture was covered by the rest 5 grams glass frit.
About 5 grams plastic balls were disposed into the bottle. The balls were prepared in such a way that the ratio of the number of big balls (¼″ dia.) to the number of small balls (⅛″) was 3:2. The bottle was set into the Turbula blender and the mixture was blended for 5 mins at 72 rpm, for example. The balls were manually sieved out by using about 1000 micron sieve screen. The blended mixture was collected using a metal pan and was put under a heating lamp for drying with a full power for a certain time (see Table 1, about 9 or 10 minutes would be the best condition.)
While being heated to dry, the mixture was stirred every one minute to ensure the entire mixture being exposed to the heat evenly. Then the separated and conglomerated glass frits were sieved out for 10 minutes. The sieve stack used in this experiment was 255 um/151 um/pan. If the mixture was sticky on the 255 micron screen, this meant that it was not dried sufficiently. In this case, the mixture was blended back and was dried under the heating lamp. After sieving, the crystals were collected on 151 micron screen and were bottled for next step process—vacuum firing. The coverage of glass frits were examined on the CBN crystals by using optical microscope. Each CBN crystal was encapsulated by the white glass frits completely.
An alternative method to remove glycol during above process steps was to pour crystals into a filtered beaker first and then evacuate the liquid glycol using vacuum pump for 1 or 2 seconds.
600 grams of Al2O3 particles (140/170 mesh) were mixed with 100 grams of the frits/CBN particles in a stainless steel splitter. Usually the ratio for Al2O3 and cBN particles was kept as 6:1. The mixture was held using a ceramic crucible, which was transferred into a vacuum furnace where the vacuum was set at least 1×10−5 torr. As soon as the furnace vacuum reached 1×10−5 torr, the heat was turned on and was set up to 350° C. for 30 minutes. Then temperature was ramped up to 900° C. at which the dwell time was set to 60 minutes. After completing the heating cycle, the temperature was ramped down by 5 degrees per minute to ensure the quality of the glass coating on the CBN crystals. The crucible was taken out of the vacuum furnace as the temperature dropped down to 50° C. The mixture was sieved carefully with 127 micron sieve screen to remove Al2O3 particles. A 107 micron sieve was used to hold the rest of remaining particles. The particles were rubbed through the sieve screen to let most of Al2O3 particles separate from CBN crystals and fall through the sieve. The rest was collected on 107 micronsieve and was put onto a sieve stack 255/181/151/pan for sieving around 15 minutes. Those crystals were collected on the 151 mesh sieve and kept in a steel container.
Those on 255 micron and 181 micron sieves were combined and put into a plastic milling jar. Ceramic balls were added into the jar and a ratio of 250 grams of ball over 50 grams of CBN/Al2O3 was kept as 5:1. A turbula milling was used for 10 mins at 72 rpm. Lastly, the Al2O3 particles were sieved out of the mixture using a sieve stack of 700 micron (remove ceramic balls)/255 um/181 um/151 um/pan. Those crystals were collected at both 181 microns and 151 microns and were put into the steel container.
Experiment 3
Alcohol was added into the steel container and any residual Al2O3 debris was decanted out. This could be performed for 3 or 5 times so as to ensure that no white debris in the glass coated cBN particles could be observed. The glass coated cBN particles were heated for 10 mins under the heating lamp. This was the final glass coated cBN product.
Experiment 4
Grinding Performance
To determine the performance of the glass coated CBN grains of the present invention in vitrified bond grinding systems, an experiment was implemented. In the experiment, two sets of grinding wheels were made by Wendt Dunnington using CBN VBR product (manufactured at Diamond Innovations, Worthington, Ohio) and a glass coated CBN VBR product described in the present invention. The condition of the wheels for both types of CBN grains were identical (see Table 2). The work pieces used for the grinding tests were M2 steel and 5150 steel, respectively. The grinding conditions are shown in Table 3.
Grinding tests were conducted at AADC (American Application Development Center) for these two sets of wheels consecutively. Creep feed grinding procedures were used to monitor radial wheel wear, grinding power, and surface finish. Grinding ratio was determined as the volume of work piece materials grounded at the threshold of required surface finish divided by volume of wheel wear and shown in
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/in/min (8.4 mm3/mm/sec.)
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While reference has been made to specific embodiments, it is apparent that other embodiments and variations can be devised by others skilled in the art without departing from their spirit and scope. The appended claims are intended to be construed to include all such embodiments and equivalent variations.
