Protective coating

1. A method for depositing a protective coating onto a substrate, wherein:

the protective coating comprises (i) a moisture-barrier layer which is in contact with the substrate and which comprises a first sub-layer, one or more intermediate sub-layers, and a final sub-layer, (ii) a mechanical-protective layer which is inorganic, and (iii) a gradient layer interposing the moisture-barrier layer and the mechanical-protective layer; and

the method comprises:

(a) depositing the first sub-layer of the moisture-barrier layer onto the substrate by plasma-deposition of a first precursor mixture comprising a fluorohydrocarbon and optionally an organosilicon compound or a compound of formula (X);

wherein:

Z 1 represents C 1 -C 3 alkyl or C 2 -C 3 alkenyl;

Z 2 represents hydrogen, C 1 -C 3 alkyl or C 2 -C 3 alkenyl;

Z 3 represents hydrogen, C 1 -C 3 alkyl or C 2 -C 3 alkenyl;

Z 4 represents hydrogen, C 1 -C 3 alkyl or C 2 -C 3 alkenyl;

Z 5 represents hydrogen, C 1 -C 3 alkyl or C 2 -C 3 alkenyl; and

Z 6 represents hydrogen, C 1 -C 3 alkyl or C 2 -C 3 alkenyl,

(b) depositing the one or more intermediate sub-layers of the moisture-barrier, each by plasma-deposition of a second precursor mixture comprising a fluorohydrocarbon or a compound of formula (X);

(c) depositing the final sub-layer of the moisture barrier layer by plasma-deposition of a third precursor mixture comprising a fluorohydrocarbon or a compound of formula (X);

(d) depositing the gradient-layer onto the final sub-layer of the moisture-barrier layer by plasma-deposition of a fourth precursor mixture which comprises the components of both the third precursor mixture and a fifth precursor mixture; and

(e) depositing the mechanical-protective layer onto the gradient-layer by plasma-deposition of the fifth precursor mixture comprising an organosilicon compound.

2. The method according to claim 1 , wherein the plasma deposition of the first precursor mixture is plasma enhanced chemical vapour deposition (PECVD).

3. The method according to claim 1 , wherein the plasma deposition of the first precursor mixture occurs at a pressure of 0.001 to 10 mbar.

4. The method according to claim 1 , wherein the organosilicon compounds are independently selected from hexamethyldisiloxane (HMDSO), tetramethyldisiloxane (TMDSO), 1,3-divinyltetramethyldisiloxane (DVTMDSO), hexavinyldisiloxane (HVDSO) allyltrimethylsilane, allyltrimethoxysilane (ATMOS), tetraethylorthosilicate (TEOS), trimethylsilane (TMS), triisopropylsilane (TiPS), trivinyl-trimethyl-cyclotrisiloxane (V 3 D 3 ), tetravinyl-tetramethyl-cyclotetrasiloxane (V 4 D 4 ), tetramethylcyclotetrasiloxane (TMCS), octamethylcyclotetrasiloxane (OMCTS), hexamethyldisilazane (HMDSN), 2,4,6-trimethyl-2,4,6-trivinylcyclotrisilazane, dimethylamino-trimethylsilane (DMATMS), bis(dimethylamino)dimethylsilane (BDMADMS) and tris(dimethylamino)methylsilane (TDMAMS).

5. The method according to claim 4 , wherein the organosilicon compounds are independently selected from hexamethyldisiloxane (HMDSO) and tetramethyldisiloxane (TMDSO).

6. The method according to claim 1 , wherein the fluorohydrocarbon compounds that may be present in any one of the first precursor mixture, the second precursor mixture, the third precursor mixture, or the fourth precursor mixture are C 3 F 6 .

7. The method according to claim 1 , wherein the mechanical-protective layer has a nanohardness of at least 6 GPa.

8. The method according to claim 1 , where the fifth precursor mixture comprises a nitrogen-containing organosilicon compound.

9. The method according to claim 1 , wherein each of the first precursor mixture, the third precursor mixture and the second precursor mixture comprise an organosilicon compound.

10. The method according to claim 1 , wherein the substrate is (i) the external casing of an electrical device, or (ii) a printed circuit board.