concatene variable and join with comma

Hello

I search to build $FROMCLAUSE DISK = N'\\nas\backup\Gdete.20160330_124543.1_3.dmp', DISK= N'\\nas\backup\Gdete.20160330_124543.2_3.dmp', DISK= N'\\nas\backup\Gdete.20160330_124543.3_3.dmp'

$DumpDir="\\nas\backup"
$BackupFileList = @(Invoke-SQLCmd -query "set nocount on; SELECT physical_device_name FROM msdb.dbo.backupmediafamily CROSS JOIN (SELECT max_value = MAX(media_set_id) FROM msdb.dbo.backupset b where b.type = 'D' and database_name = 'gdete') m WHERE media_set_id = max_value" -Server test\srv)
Foreach($file in $BackupFileList){$DumpfileList += @([System.IO.Path]::GetFileName($file.physical_device_name))}
The result is
Gdete.20160330_124543.1_3.dmp
Gdete.20160330_124543.2_3.dmp
Gdete.20160330_124543.3_3.dmp

I try this :
$FROMCLAUSE=($DumpfileList  |Select-object @{name="file.physical_device_name";expression={"DISK = N'"+$dumpdir+$file.physical_device_name +"'"}} | Select -ExpandProperty file.physical_device_name) -join ","

But the result is not correct

DISK = N'\\nas\backupN:\MSSQL\backup\Gdat.20160330_124543.3_3.dmp',DISK = N'\\nas\backupN:\MSSQL\backup\Gdat.20160330_124543.3_3.dmp', DISK = N'\\nas\backupN:\MSSQL\backup\Gdat.20160330_124543.3_3.dmp'

How can I resolve ?

Thanks

Regards