Claims:WE CLAIM:

1. Zirconia doped MgAl2O4 spinel ceramics with enhanced properties comprisingnano-carbon as an added component.

2. Ceramics as claimed in claim1, wherein the nano-carbon content in the initial composition ranges between 0.5% to 3% with particle size ranging between 10nm to 100nm.

3. Ceramics as claimed in claims 1 and 2, wherein the particle size of nano -carbon ranges between 20nm to 90nm.

4. Ceramics as claimed in claims 1 to 3, wherein the particle size of nano- carbon is 20and/or 90nm.

5. Ceramics as claimed in claims 1 to 4, wherein initialZirconia content in monoclinic form is between 20 to 25% with particle size ranging between 4 to 6 micrometer and particle size of spinel ranges between 5 to 10 micrometer.

6. Ceramics as claimed in claims 1 to 5, wherein initial Zirconia content in monoclinic form is about 20%.

7. Ceramics as claimed in claims 1 to 6, whereinthe enhanced properties include further increase in mechanical strength and increase inbulk densityof the ceramics at higher temperatures with decrease in apparent porosity.

8. A process for preparing Zirconia doped MgAl2O4spinel ceramics withenhanced propertiesas claimed in claims 1 to 7, wherein nano-carbon is added in thecomposition as an additional component.

9. A process as claimed in claim 8, wherein the nano-carbon content in the composition is between 0.5% to 3% with particle size ranging between 10nm to 100nm.

10. A process as claimed in claim 9, wherein the particle size of nano-carbon is 20nm and/or 90nm.

11. A process as claimed in claims 8 to 10, wherein decrease in the size of nano-
carbon would entail increase in the transformation of Zirconia from monoclinic to
tetragonal resulting in more toughening of spinel Zirconia system.

12. Ceramics articles of varying utilities whenever prepared by the process of claims
8 to 11.

Dated: this 26th day of November, 2016.

(N. K. Gupta)
Patent Agent
Of NICHE
For SAIL

To,
The Controller of Patents,
The Patent Office, Kolkata.

, Description:ZIRCONIA DOPED MgAl2O4 SPINELCERAMICS CONTAINING NANO-CARBON WITH ENHANCED PROPERTIES AND A PROCESS FOR PREPARING THE SAME

FIELD OF THE INVENTION
This invention relates to Zirconia doped MgAl2O4 spinel ceramics. This invention particularly relates to a composition of Zirconia doped MgAl2O4 spinel wherein nano-carbon is added to get ceramics with enhanced properties.

BACKGROUND OF THE INVENTION
Zirconia containing ceramics have exceptionally high fracture toughness compared with other structural ceramics. Application areas vary widely from traditional uses like refractory and glass to the more advanced fields like ball-bearing, automobile appliances, turbine blades, grinding media, cutting tools, space shuttles to bio-implantations. It has the potential to toughen and / or strengthen ceramic matrix utilising the stress-induced tetragonal to monoclinic phase transformation of ZrO2 .In the present work nano carbon is added in monoclinic zirconia doped preformed MgAl2O4 Spinel powder for the synthesis of an improved ceramics with enhanced properties.
There has been no relevant prior published report of any such work in the literature.

OBJECT OF THE INVENTION
The object of this invention is to provide Zirconia doped MgAl2O4 spinel ceramics with enhanced properties by adding nano-carbon in the composition. The enhanced properties include further toughening/strengthening of the ceramic matrix. The said object is fulfilled by the use of the invention disclosed.

BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS
Fig.1 illustrates the X Ray diffraction of Spinel Zirconia at different temperatures.
Fig. 2 illustrates the X Ray diffraction of Spinel Zirconia with course nano-carbon fired at different temperatures.
Fig. 3 illustrates the X Ray diffraction of spinel Zirconia with fine nano-carbon at different temperatures.
Fig. 4(a) illustratesthe microstructure of spinel Zirconia composition without any nano-carbon.
Fig. 4(b) illustrates the microstructure of spinel Zirconia composition with coarse nano-carbon.
Fig. 4(c) illustrates the microstructure of spinel Zirconia composition with fine nano-carbon

DESCRIPTION OF THE INVENTION
According to this invention there is provided Zirconia doped MgAl2O4 spinel ceramics with enhanced properties which comprises nano-carbon as an additional component in the initial composition. Thenano-carbon content in the composition usedis between 0.5% to 3% with particle size ranging between 10nm to 100nm.
In a preferred embodiment the particle size of nano -carbon is kept in the ranges between 20nm to 90nm. According to another preferred embodiment the particle size of nano- carbon is kept at 20nm and/or 90nm.

In the ceramics of the invention initial Zirconia content is in monoclinic form and is kept between 20 to 25% with particle size ranging between 4 to 6 micrometer and particle size of spinelis kept in the ranges between 5 to 10 micrometer. According to a preferred embodimentinitial Zirconia content in monoclinic form is kept at or about 20%.

The enhanced properties include further increase in mechanical strength and increase in bulk density of the ceramics at higher temperatures with decrease in apparent porosity.

According to this invention there isalso provided a process for preparing Zirconia doped MgAl2O4 spinel ceramics with enhanced properties wherein nano-carbon is added in the initial composition to prepare the ceramics.While preparing the invented ceramics the nano-carbon content in the composition is kept between 0.5 to 3% with particle size ranging between 10 to 100nm,preferablybetween 20 to 90nm.

According to a preferred embodiment of theinvented process the particle size of nano-carbon used is 20nm and/or 90nm.

The invention also includes ceramics articles of varying utilities in different fields of technology whenever prepared by the above process.
Zirconia containing ceramics have exceptionally high fracture toughness compared with other structural ceramics. Application areas vary widely from traditional uses like refractory and glass,castables, coating making to the more advanced fields like ball-bearing, automobile appliances, turbine blades, grinding media, cutting tools, space shuttles to bio-implantations. It has the potential to toughen and / or strengthen ceramic matrix utilizing the stress-induced tetragonal to monoclinic phase transformation of ZrO2.

Process of manufacture:
Two sizes of nano carbon black were taken whose mean particle sizewas 20nm and 90nm. The particle size of spinel powder was 8.54 µm and that of zirconia was 5.67 µm. The composition is mentioned below in Table-1.

Table – 1
Composition
Sample No. Spinel(gm) ZrO2(gm) Carbon black(90nm) in gm Carbon black(20nm) in gm %
Of carbon addition
Sample 1 60 15 - - 0%
Sample 2 60 15 0.375 - 0.5%
Sample 3 60 15 0.75 - 1%
Sample 4 60 15 1.5 - 2%
Sample 5
Sample 6 60
60 15
15 -
- 0.75
1.5 1%
2%

Different compositions were mixed in a centrifugal mixer by moving it clock and anti clock direction alternately by 30 sec at 40 rpm for 30 minutes.Samples were made of 30mm dia. and 50mm X 10mm rectangular mould with the help of Carver lab press at a pressure of 10 kg/cm2.All compositions were heated at a fixed heating rate of 10oC/minupto 1200, 1350 and 1500oC for2 hours in a chamber furnace in normal atmosphere.

Characterization:
The physical parameters like bulk density and apparent porosity were measured by using Archimedes' principle in kerosene oil. The bulk densityincreased gradually from 2.72 gm/cm3 of Spinel with20% monoclinic zirconia at 1200oC/2h to 2.76 and 3.04 gm/cm3 without nano carbon added sample at 1350oC and 1500oC for 2 hour respectively as shown in Table 2.

Table-2
Bulk density of different spinel zirconia composition with different size of nano carbon
sample 12OO◦c /2h 1350◦c/2h 1500/2h

sam1 2.72 2.76 3.04
sam2 2.70 2.76 3.04
sam3 2.74 2.79 3.074
sam4 2.66 2.75 2.93
sam5 3.03 3.02 3.12
sam6 2.80 3.0 3.16

The apparent porosity of spinelalso decreased from 32.29% at 1200oC/2h to 29.78 & 25.26% at 1350, 1500oC/2h respectively with 20 wt.% zirconia addition as shown in Table 3.

Table-3
Apparent porosity of different spinel zirconia composition with different size of nano carbon

sample 1200◦c/2h 1350◦c/2h 1500◦c/2h
sam1 28.59 26.02 20.65
sam2 29.23 26.92 20.52
sam3 28.63 26.75 20.74
sam4 30.09 26.76 23.90
sam5 29.25 22.22 17.14
sam6 28.86 22.89 19.03

With nano carbon black of 90nm size addition in spinel – 20% zirconia compacts the bulk density increased gradually from 2.74 gm/cm3 at 1200oC/2h to 2.79 and 3.08 gm/cm3 at 1350oC and 1500oC for 2 hour respectively. The apparent porosity also decreased from 28.03% at 1200oC/2h to 26.02& 20.65% at 1350, 1500oC/2h respectively with 20 wt.% zirconia addition.With nano carbon black of 20nm size addition in spinel – 20% zirconia compact the bulk density increased gradually from 2.85 gm/cm3 at 1200oC/2h to 3.02 and 3.12 gm/cm3 at 1350oC and 1500oC for 2 hour respectively. The apparent porosity also decreased from 25.04% at 1200oC/2h to 22.1&17.65% at 1350, 1500oC/2h respectively with fine 20nm 1% carbon black addition.Whereas by an increase of coarse nano carbon(90 nm) to 2% the bulk density was increased compared to 1% carbon addition. At the same time increase of fine (20 nm) carbon black addition also made the bulk density increased withdecrease of apparent porosity.

From the XRD it isobserved that in spinel-zirconia compound there is some transformation of monoclinic zirconia to tetragonal phase but no new compound is formed as shown in Fig.1. Whereas at higher temperature at 1500◦c/2h the transformation of tetragonal phase slightly decreases as well as mechanical strength also decreases without any addition of carbon.

In presence of (90nm size) carbon the transformation to tetragonal phase increases but the quantity of this carbon also restricts the transformation slightly (Fig. 2) whereas in presence of fine (20 nm size) nano- carbon, the transformation to tetragonal phase was much more than as seen in case of coarse (90nm size) nano- carbon(Fig. 3). It may be due to more distribution of carbon into the matrix.

The different phases present after firing at different temperaturesaresummarised and presented in the Table-4.

Table-4
Phases present of Spinel- Zirconia – NC Compacts after firing at different temperatures
At 1200◦c/2h
sam1 sam2 sam3 sam4 sam5 sam6
Spinel 84.27% 84.54% 85.98% 85.85% 84.81% 83.05%
ZrO2 monoclinic 12.88% 11.89% 11.63% 11.30% 7.86% 9.12%
ZrO2 tetragonal 2.85% 3.58% 2.39% 2.85% 7.33% 7.83%

At 1350◦c/2h
sam1 sam2 sam3 sam4 sam5 sam6
Spinel 84.62% 82.69% 85.97% 82.78% 82.89% 82.63%
ZrO2 monoclinic 11.58% 12.54% 9.51% 12.27% 10.00% 9.87%
ZrO2 tetragonal 3.80% 4.77% 4.52% 4.95% 7.11% 7.51%

At1500◦c/2h
sam1 sam2 sam3 sam4 sam5 sam6
Spinel 85.08% 83.40% 86.10% 84.98% 84.41% 83.80%
ZrO2 monoclinic 11.75% 11.59% 10.94% 11.01% 10.63% 9.47%
ZrO2 tetragonal 3.17% 5.01% 2.96% 4.01% 4.96% 6.73%

The mechanical strength of the spinel increased gradually with increase in Zirconia content and temperature. With addition of coarse nano-carbon the mechanical strength increased to some extent while with fine nano carbon the mechanical strength increased gradually with increase in temperature. The compressive strength increased from 15.7MPa with 25wt% zirconia at 1200oC to40.4MPa at 1500oC as shown in Table-5.With coarse nano carbon the mechanical strength slightly increased with increased in temperature to 45.8MPa. But with fine nano carbon the strength gradually increased with increased in temperature up to 54.7MPa at 1500oC.
Table -5
Compressive strength of Spinel Zirconia Nano carbon fired at different temp.
sample 1200◦c/2h 1350◦c/2h 1500◦c/2h

sam1 15.7 27.3 40.4
sam2 16.8 30.1 41.8
sam3 16.7 32.3 42.7
sam4 18.3 33.1 45.8
sam5 18.5 36.2 53.2
sam6 19.1 38.4 54.7

The microstructure of the composition without nano carbon and with coarse and fine nano carbon also confirming the distribution of zirconia is better with fine nano carbon at higher temperature is shown in Fig. 4(a-c).

Salient features of theinvention:
- Zirconia increased the mechanical strength of spinel due to transformation of monoclinic to tetragonal phase.
- Nano carbon black fill the gap in the compacts.
- Nano carbon material carbon black of different sizes affects the transformation of monoclinic to tetragonal phase that also enhance the mechanical strength.
- Sizes of carbon black affect toughening of spinel zirconia compacts
- In presence of 1-2 % fine nano carbon black (20nm) transformation of zirconia from monoclinic to tetragonal is more than coarse nano carbon (90nm) addition. The toughening of spinel is also increased that results in more mechanical strength.
-
From the above disclosure of the invention it is apparent that several additional embodiments beyond those disclosed herein are possible which can be carried out by a person skilled in the art and the same are included within the broad ambit of the invention claimed herein.