DESC:TITLE OF THE INVENTION: CERAMIC-COATED ABRASIVE GRAINS AND A PROCESS OF MANUFACTURING THE SAME
FIELD OF THE INVENTION
The present disclosure is generally related to abrasive grains. Particularly, the present disclosure is related to ceramic-coated abrasive grains. More particularly, the present disclosure is related to: ceramic-coated abrasive grains; and a process of manufacturing the same.
BACKGROUND OF THE INVENTION
Several types of alumina-based abrasives are widely used, in the industry. Typical applications of these grains are in: coated abrasives; bonded abrasives; loose abrasives; and/or blasting applications; end products are machined, ground, shaped, and/or polished, depending on requirements. In this connection, BFA grains, WFA grains, AZ grains, and sintered alumina grains, derived by sol-gel processing (SG), are known in the art.
Primary parameters that are of importance, from a user point of view, include: MRR, which denotes material removal rate, from a job, without affecting finally required finish; and GR, which represents ratio between weight of material removed, from the job, to loss of grinding tool.
High GR and MRR translate to economically favourable operations, due to low consumable costs and better production rates. Though different grains perform differently, with different job materials (such as mild steel, stainless steel, aluminium, and/or the like), it is known in the art that performances are impacted, by: toughness; hardness; and/or friability of the grains.
Friability of the grains is often used, as a direct measure of performance. One way to decrease the friability of the grains (i.e. increase resistance to fracture) is by cementing surface micro-cracks. Cracks inside the grains and on their surface will increase the friability of the grains. Alternate approaches include increasing the toughness of the grains, by microstructural refinements.
To improve performances of the grains, surface roughening is also seen as a potential method, through better resin bonding and reduced shedding. Several approaches have been documented, in the past, but most of them indicate that improvement of one property leads to a deterioration of another.
There is, therefore, a need in the art, for: a ceramic coating; ceramic-coated abrasive grains; and a process of manufacturing the same, which: decrease the friability of the grains; and increase the adhesion of the grains, thereby, aiding, in higher GR and MRR.
SUMMARY OF THE INVENTION
A ceramic coating, for abrasive grains, is disclosed. Said ceramic coating broadly comprises: orthophosphoric acid; alumina powder; aluminium metal powder; and low temperature frit or medium temperature frit.
Concentration of said orthophosphoric acid is about 2% by weight.
Concentration of said alumina powder is about 5% by weight.
Concentration of said aluminium metal powder is about 0.5% by weight.
Concentration of said low temperature frit or said medium temperature frit is about 1% by weight.
A process of manufacturing a ceramic coating, for abrasive grains, broadly comprises following steps:
First, orthophosphoric acid, alumina powder, aluminium metal powder, and low temperature frit or medium temperature frit, are mixed, with a base abrasive grain (or a bare abrasive grain, or a raw abrasive grain), to form a mixture.
Concentration of said orthophosphoric acid is about 2% by weight.
Concentration of said alumina powder is about 5% by weight.
Concentration of said aluminium metal powder is about 0.5% by weight.
Concentration of said low temperature frit or said medium temperature frit is about 1% by weight.
Said mixture is dried, at about 650 degrees Centigrade or about 850 degrees Centigrade, in case of said low temperature frit or said medium temperature frit, respectively, to generate micro bubbles, with diameter of said micro bubbles ranging between about 50 µm and about 1,000 µm. Heating rate is about 5 degrees Centigrade per minute.
Finally, to break said micro bubbles, and to form a layered ceramic coating, cooling is performed, in a furnace. Cooling rate ranges between about 25 degrees Centigrade per minute and about 50 degrees Centigrade per minute.
Ceramic-coated abrasive grains, and a process of manufacturing the same, are also disclosed.
The disclosed ceramic coating, ceramic-coated abrasive grains, and process of manufacturing, offer at least the following synergistic advantages and effects: are cost-effective; decrease the friability of the grains; increase the adhesion of the grains; and aid, in achieving high grinding ratios and material removal rates.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1(a) and Figure 1(b) illustrate a BFA grain, without a ceramic coating, and with a ceramic coating, respectively, in accordance with an embodiment of the present disclosure;
Figure 2(a) and Figure 2(b) illustrate an AZ grain, without a ceramic coating, and with a ceramic coating, respectively, in accordance with an embodiment of the present disclosure;
Figure 3 illustrates a secondary electron microscopic image of a BFA grain, coated with a ceramic coating, in accordance with an embodiment of the present disclosure;
Figure 4(a) illustrates a bare grain (or a bare abrasive grain), in accordance with an embodiment of the present disclosure;
Figure 4(b) illustrates a grain, coated sequentially with a glassy amorphous binder and a phosphatic binder, in accordance with an embodiment of the present disclosure;
Figure 4(c) illustrates a grain, coated sequentially with a phosphatic bond material and a glassy amorphous binder, in accordance with an embodiment of the present disclosure;
Figure 4(d) illustrates a grain, coated with both a phosphatic bond material and a glassy amorphous binder, simultaneously, in accordance with an embodiment of the present disclosure;
Figure 5 illustrates a performance chart of: a bare grain (or a bare abrasive grain); and a grain, coated with a ceramic coating, in cut-off wheels (resinoid wheels), in accordance with an embodiment of the present disclosure;
Figure 6 illustrates process flow involved, during process of manufacturing ceramic-coated abrasive grains, in accordance with various embodiments of the present disclosure;
Figure 7 illustrates complete covering of a ceramic-coated abrasive grain’s surface, with frit, in accordance with an embodiment of the present disclosure; and
Figure 8 illustrates formation of micro bubbles, during process of manufacturing ceramic-coated abrasive grains, in accordance with various embodiments of the present disclosure
DETAILED DESCRIPTION OF THE INVENTION
Throughout this specification, the use of the words “comprise” and “include”, and variations, such as “comprises”, “comprising”, “includes”, and “including”, may imply the inclusion of an element (or elements) not specifically recited. Further, the disclosed embodiments may be embodied, in various other forms, as well.
Throughout this specification, the use of the word “additives” is to be construed as: “OPA; and/or Alumina Powder; and/or Aluminium Metal Powder; and/or Red Iron Oxide; and/or LT Frit; and/or MT Frit; and/or the like”.
Throughout this specification, the use of the phrase “base abrasive grain”, and its variations, is to be construed as: “BFA abrasive grains; AZ abrasive grains; and/or the like, as the case may be”.
Throughout this specification, the use of the acronym “BFA” is to be construed as: “Brown Fused Alumina”.
Throughout this specification, the use of the acronym “WFA” is to be construed as: “White Fused Alumina”.
Throughout this specification, the use of the acronym “AZ” is to be construed as: “Fused Alumina Zirconia”.
Throughout this specification, the use of the acronym “MRR” is to be construed as: “Material Removal Rate”.
Throughout this specification, the use of the acronym “GR” is to be construed as: “Grinding Ratio”.
Throughout this specification, the use of the acronym “SF” is to be construed as: “Semifriable”.
Throughout this specification, the use of the acronym “FEPA” is to be construed as “Federation of European Producers of Abrasives”.
Throughout this specification, the use of the acronym “OPA” is to be construed as: “Orthophosphoric Acid”.
Throughout this specification, the use of the acronym “MT” is to be construed as: “Medium Temperature”.
Throughout this specification, the use of the acronym “LT” is to be construed as: “Low Temperature”.
Throughout this specification, the use of the acronym “Na2O” is to be construed as: “Sodium Oxide”.
Throughout this specification, the use of the acronym “B2O3” is to be construed as: “Boron Trioxide”.
Throughout this specification, the use of the acronym “Al2O3” is to be construed as: “Aluminium Oxide”.
Throughout this specification, the use of the acronym “SiO2” is to be construed as: “Silicon Dioxide”.
Throughout this specification, the use of the acronym “Fe2O3” is to be construed as: “Ferric Oxide”.
Throughout this specification, the use of the acronym “ZrO2” is to be construed as: “Zirconium Dioxide”.
Throughout this specification, the use of the acronym “TiO2” is to be construed as: “Titanium Dioxide”.
Throughout this specification, the disclosure of a range is to be construed as being inclusive of: the lower limit of the range; and the upper limit of the range.
Throughout this specification, where applicable, the word “grain” and the phrase “abrasive grain” are used interchangeably.
Throughout this specification, the words “the” and “said” are used interchangeably.
Also, it is to be noted that embodiments may be described as a method. Although the operations, in a method, are described as a sequential process, many of the operations may be performed in parallel, concurrently, or simultaneously. In addition, the order of the operations may be re-arranged. A method may be terminated, when its operations are completed, but may also have additional steps.
Ceramic-coated abrasive grains, with improved performances, by providing a hard shell around the material, which resist chipping of the grains (also referred to as “ceramic-coated abrasive grains” and/or “grains”), is disclosed.
A combination of phosphatic and glassy coating (also referred to as “ceramic coating” and/or “coating”), of specific compositions, drastically improves toughness and surface roughness, and, hence, performances. Formation of a glassy surface and phosphatic interface increases: adhesion of the grains; capillarity of the grains; and wettability of the grains, simultaneously.
The ceramic-coated abrasive grains offer low friability and high adhesion, thereby, aiding, in higher GR and MRR. In an embodiment of the present disclosure, said ceramic-coated abrasive grains broadly comprise: a fine ground ceramic material; and/or a glassy amorphous binder (also referred to as “glassy binder”; for example, frit); and/or a phosphatic bond material (for example, phosphoric acid or phosphate); and/or a ceramic powder of coarser size; and/or a metal powder catalyst.
In another embodiment of the present disclosure, process of manufacturing the ceramic-coated abrasive grains broadly comprises mixing (i) the fine ground ceramic material; and/or (ii) the glassy amorphous binder; and/or (iii) the phosphatic bond material; and/or (iv) the ceramic powder of coarser size; and/or (v) the metal powder catalyst, with a base abrasive grain (or a bare abrasive grain, or a raw abrasive grain), by rotary motion, followed by heating, to high temperatures.
The coating thus formed is a reinforced composite of polycrystalline phosphate, with an amorphous glassy phase and splat-like structures uniformly covering the grains; surface cracks in the grains that are generated, by crushing and grading, are filled (or cemented), with the phosphatic bond material, whilst the low viscosity glassy phase enhances the coating, by forming barriers, to crack propagation. This is reflected, by a drastic decrease, in the friability of the grains.
The disclosed ceramic-coated abrasive grains are cured, at high temperatures, to form a hard layer, in presence of a catalyst, and, thereby, seal surface cracks and pores. The coating is engineered to have a discontinuous layered composite structure that dramatically enhances the toughness of the grains, and, hence, their performances.
In yet another embodiment of the present disclosure, the process of manufacturing the ceramic-coated abrasive grains is carried out at a temperature of about 700 degrees Centigrade or below, thereby, being useful for Fused Alumina Zirconia grains, which degrade during high temperature treatments (above about 900 degrees Centigrade).
In yet another embodiment of the present disclosure, a reactive form of alumina, along with phosphoric acid, forms the phosphatic bond material. The alumina is nanostructured, eutectic alumina-zirconia, with microparticles coated on the surface, having a higher toughness than the base abrasive grains (or the bare abrasive grains, or the raw abrasive grains).
The ceramic-coated abrasive grains were synthesised, and tested, as follows:
The raw materials used were as follows:

Figure 6 illustrates the process flow involved. Grit size ranged from F 6 to 220, for case-bonded applications, and from P 12 to 220, for coated applications. Heating rate was about 5 degrees Centigrade/minute.
As illustrated, in Figure 7, the frit was determined to melt and completely cover the surface of the grains, at between about 600 degrees Centigrade and about 700 degrees Centigrade, in case of LT frit. Additives were determined to be embedded, with the glassy binder, and coated on the surface of the grains.
Micro bubbles were determined to collapse, during cooling period, to create an island kind of coating structure (layered coatings). To understand further, the LT frit was heated, to about 600 degrees Centigrade, and cooled down, to observe the micro bubble formation (between about 50 µm and about 1,000 µm, in diameter, as illustrated, in Figure 8).
However, to break the bubbles and generate splat morphology, furnace cooling rate ranged between about 25 degrees Centigrade/minute and about 50 degrees Centigrade/minute. This resulted in a layered coating, of: length that ranged between about 1 µm and about 10 µm; thickness that ranged between about 0.5 µm and about 5 µm; and width that ranged between about 1 µm and about 10 µm.
EXAMPLE 1
SF grains are fused aluminium oxide, with about 1.5% titanium, and are used widely. in the industry. These grains were heat treated, after mixing with the OPA and/or the frit, and tested for friability. The base grains (or the bare grains, or the raw grains) were of F60 grit, with size distribution, as specified by FEPA.
About 250 grams of the F60 was prepared, according to FEPA standards. It was sieved, through a mesh (#60, as per ASTM standards), and about 100 grams of + 60 size material was collected. The sieved material was kept, in a ball mill lined with chrome steel, and subjected to ball milling, with steel balls, having diameter of about 1.25 cm. Total weight of the balls was about 1.5 kg, and the material was milled, for about 950 rotations. The obtained milled powder was sieved, based on FEPA specifications; material passing through #70 mesh ASTM sieves was weighed, and the friability was calculated, as follows:
Friability (%) = (Weight of Material Obtained from #70 Mesh Sieve/Total Weight of Material) x 100
The friability values of different samples were determined, as shown below (component percentages are percentages by weight).
Sample Code OPA (%) Alumina Powder Aluminium Metal Powder Red Iron Oxide LT frit (%) Friability (%)
RAW SF 0 0 0 0 0 43
A 2 0 0 0 0 28
B 0 0 0 0 1 28
C 2 0 0 0 1 28
D 2 5 0.5 0 1 23
E 2 5 0.5 0.5 1 23
F 4 0 0.8 0.8 1 32
G 4 5 0.8 0.8 1 23

After mixing with a desired binder, heat treatment was done, at about 650 degrees Centigrade, for about 30 minutes. There was a reduction, in the friability, with addition of the OPA (Sample A) and the LT frit (Sample B). This was attributed to filling up of the phosphate, in the cracks formed, on the surface of the grains, in case of the OPA.
In case of the frit, it melts at temperatures of about 650 degrees Centigrade, and depth of penetration inside the pores is limited, by viscosity of the frit. It should be noted that frits have different melting points, depending on composition, and the LT frit involved here was a low temperature frit, with a softening point of about 600 degrees Centigrade. From samples A and B, it was confirmed that each of these additives, individually, has an ability to reduce the friability.
However, surprisingly, it was found that the mere addition of both the additives, in Sample C, and processing of the same, under similar conditions, did not reduce the friability further.
In Sample D, additional ingredients (such as alumina powder and aluminium metal powder) were added, before the heat treatment. In this case, the friability dropped further, indicating that synergistic effects of the OPA and the LT frit is realised, only when the metal powder catalyst is present, and reactivity is enhanced, by addition of the alumina powder.
No further improvements, in adhesion of grains, were achieved, with addition of red iron oxide powder, as shown, in Sample E. Sample F and Sample G compare effects of higher quantities of the OPA, in the mix. Similar to the results discussed earlier, the coating does not impart improved toughness (to the grains), when the alumina powder is not included.
Figure 1(a) and Figure 1(b) illustrate a BFA grain, without the ceramic coating, and with the ceramic coating, respectively.
EXAMPLE 2
Coating trials were performed, to understand mechanisms and efficiencies, in improving the friability of the grains, as shown, in the below Table. Here, a MT frit, with a softening point of about 850 degrees Centigrade, was involved.
Instead of the alumina powder, AZ fines (sizes finer than about 325 mesh) were involved. The base grains (or the bare grains, or the raw grains) tested were BFA, with an initial friability of about 38%. Subsequent to the tests, the friability dropped, by more than about 50%, as shown below (component percentages are percentages by weight).
Sample Code OPA
(%) Alumina Powder Aluminium Metal Powder Fused Alumina Zirconia Fines MT Frit (%) Friability (%)
RAW BFA 0 0 0 0 0 38
H 2 0 0.5 0 1 25
I 2 0 0.5 5 1 18

EXAMPLE 3
For all cases, from Samples A to I, a mix was made, in a planetary mixer (or pan mixer), to create homogeneity. To ascertain mechanisms, controlled trials were performed, with ingredients being coated, in a sequential manner. The base grains (or the bare grains, or the raw grains) were AZ eutectic, with about 42% by weight of zirconia and grit size FEPA P50.
Sample J was formed, by coating with the phosphatic bond material first, and then with the frit. The sequence was reversed, in case of Sample K. After each coating, there was an about 650 degrees Centigrade heat treatment, to cure binding layers.
In Sample L, all four ingredients were mixed together, along with the grains, and a single step high temperature treatment, at about 650 degrees Centigrade, was done.
The results obtained were as follows:
Material First Coat Second Coat Friability %
Raw AZ P50 Nil Nil 26
J OPA + Alumina Powder + Aluminium Metal Powder Frit 18
K Frit OPA + Alumina Powder + Aluminium Metal Powder 17
L OPA + Alumina Powder + Aluminium Metal Powder + Frit Nil 12

From the friability values, it was determined that the most efficient coating was that of the Sample L. Figure 2(a) and Figure 2(b) illustrate an AZ grain, without a ceramic coating, and with a ceramic coating, respectively.
Cross Sectional Analysis of the Coating:
The coated grains were also dissected, with a focused ion beam, observed under a microscope, and found to have a splat-like microstructure, as illustrated, in Figure 3. The discontinuous structure acts as crack deflector, and, hence, improves the toughness of the grains.
Figure 4(a) illustrates a graphical representation of a bare grain (or a base grain, or a raw grain), while Figure 4(b) illustrates a graphical representation of a grain, coated sequentially with the glassy amorphous binder first. Figure 4(c) illustrates a graphical representation of a grain, coated sequentially with the phosphatic bond material first. Figure 4(d) illustrates a graphical representation of a grain, coated with both the phosphatic bond material and the glassy amorphous binder, simultaneously.
EXAMPLE 4
Cut-off wheels, of about 1 mm thickness, were made, using AZ grains, with and without the coating, as per Example 3 (Sample L). Cutting ratios were determined to be improved, by more than about 15%. Cutting times were also determined to be reduced. As illustrated, in Figure 5, As illustrated, in Figure 5, the coating was determined to be effective (in improving the performances of the grains)..
The ceramic-coated BFA abrasive grains showed an about 100% increase, in GR, when tested, in resinoid wheels. In case of the AZ eutectic ceramics, the wheel life was increased, by more than about 30%.
The present disclosure is to be construed as also disclosing the coating, which is coated, onto the abrasive grains, as explained above.
The disclosed ceramic coating, ceramic-coated abrasive grains, and process of manufacturing, offer at least the following synergistic advantages and effects: are cost-effective; decrease the friability of the grains; increase the adhesion of the grains; and aid, in achieving high GR and MRR.
It will be apparent to a person skilled in the art that the above description is for illustrative purposes only and should not be considered as limiting. Various modifications, additions, alterations, and improvements, without deviating from the spirit and the scope of the disclosure, may be made, by a person skilled in the art. Such modifications, additions, alterations, and improvements, should be construed as being within the scope of this disclosure.
,CLAIMS:1. A ceramic coating, for abrasive grains, said ceramic coating: decreasing friability of said abrasive grains; increasing adhesion of said grains; aiding, in achieving high grinding ratios and material removal rates; and comprising:
orthophosphoric acid, with concentration of said orthophosphoric acid being 2% by weight;
alumina powder, with concentration of said alumina powder being 5% by weight;
aluminium metal powder, with concentration of said aluminium metal powder being 0.5% by weight; and
low temperature frit or medium temperature frit, with concentration of said low temperature frit or said medium temperature frit being 1% by weight.
2. A process of manufacturing a ceramic coating, for abrasive grains, said process: decreasing friability of said abrasive grains; increasing adhesion of said grains; aiding, in achieving high grinding ratios and material removal rates; and comprising steps of:
mixing orthophosphoric acid, alumina powder, aluminium metal powder, and low temperature frit or medium temperature frit, with a base abrasive grain, with:
concentration of said orthophosphoric acid being 2% by weight;
concentration of said alumina powder being 5% by weight;
concentration of said aluminium metal powder being 0.5% by weight; and
concentration of said low temperature frit or said medium temperature frit being 1% by weight;
drying, at 110 degrees Centigrade, for a time that ranges between four hours and eight hours;
heat treating, at 650 degrees Centigrade or 850 degrees Centigrade, in case of said low temperature frit or said medium temperature frit, respectively, to generate micro bubbles, with:
heating rate being 5 degrees Centigrade per minute; and
diameter of said micro bubbles ranging between 50 µm and 1,000 µm; and
cooling, in a furnace, to break said micro bubbles, and form said ceramic coating, with:
cooling rate ranging between 25 degrees Centigrade per minute and 50 degrees Centigrade per minute.
3. Ceramic-coated abrasive grain, said ceramic-coated abrasive grains: offering decreased friability; offering increased adhesion; offering high grinding ratios and material removal rates; and comprising:
orthophosphoric acid, with concentration of said orthophosphoric acid being 2% by weight;
alumina powder, with concentration of said alumina powder being 5% by weight;
aluminium metal powder, with concentration of said aluminium metal powder being 0.5% by weight; and
low temperature frit or medium temperature frit, with concentration of said low temperature frit or said medium temperature frit being 1% by weight.
4. A process of manufacturing ceramic-coated abrasive grains, said process: decreasing friability of said abrasive grains; increasing adhesion of said grains; aiding, in achieving high grinding ratios and material removal rates; and comprising steps of:
mixing orthophosphoric acid, alumina powder, aluminium metal powder, and low temperature frit or medium temperature frit, with a base abrasive grain, with:
concentration of said orthophosphoric acid being 2% by weight;
concentration of said alumina powder being 5% by weight;
concentration of said aluminium metal powder being 0.5% by weight; and
concentration of said low temperature frit or said medium temperature frit being 1% by weight;
drying, at 110 degrees Centigrade, for a time that ranges between four hours and eight hours;
heat treating, at 650 degrees Centigrade or 850 degrees Centigrade, in case of said low temperature frit or said medium temperature frit, respectively, to generate micro bubbles, with:
heating rate being 5 degrees Centigrade per minute; and
diameter of said micro bubbles ranging between 50 µm and 1,000 µm; and
cooling, in a furnace, to break said micro bubbles, and form said ceramic coating, with:
cooling rate ranging between 25 degrees Centigrade per minute and 50 degrees Centigrade per minute.